feat(lang): add Lean 4 support (#4080)

## Description

<!-- Describe the big picture of your changes to communicate to the
maintainers
  why we should accept this pull request. -->

Adds language support for lean, a popular proof assistant and theorem
prover.

## Related Issue(s)

<!--
  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
-->

None

## Screenshots

<!-- Add screenshots of the changes if applicable. -->

Language Server

<img width="1510" alt="lsp"
src="https://github.com/user-attachments/assets/28c29cae-eca4-4dfa-a4a4-462551f5cfd8">

Lean Execution (infobox on the right)

<img width="1510" alt="goals"
src="https://github.com/user-attachments/assets/e195dbeb-70d6-4f09-90bf-dbe289423bec">

Keymaps

<img width="1510" alt="keymaps"
src="https://github.com/user-attachments/assets/2451c3d2-b855-4fb5-9d30-33305274c52c">

Many More Commands

<img width="1510" alt="lots of commands"
src="https://github.com/user-attachments/assets/e7b7eb1d-0f4e-4d3c-a58b-a87eb7e0319c">

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

---------

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/lazyvim/plugins/extras/lang/lean.lua

@@ -0,0 +1,125 @@
+return {
+  recommended = function()
+    return LazyVim.extras.wants({
+      ft = { "lean" },
+      root = { "lean-toolchain" },
+    })
+  end,
+  "Julian/lean.nvim",
+  event = { "BufReadPre *.lean", "BufNewFile *.lean" },
+  dependencies = {
+    "nvim-lua/plenary.nvim",
+  },
+
+  -- see details below for full configuration options
+  opts = {
+    -- Enable the Lean language server(s)?
+    --
+    -- false to disable, otherwise should be a table of options to pass to `leanls`
+    --
+    -- See https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#leanls for details.
+    -- In particular ensure you have followed instructions setting up a callback
+    -- for `LspAttach` which sets your key bindings!
+    lsp = {
+      init_options = {
+        -- See Lean.Lsp.InitializationOptions for details and further options.
+
+        -- Time (in milliseconds) which must pass since latest edit until elaboration begins.
+        -- Lower values may make editing feel faster at the cost of higher CPU usage.
+        -- Note that lean.nvim changes the Lean default for this value!
+        editDelay = 0,
+
+        -- Whether to signal that widgets are supported.
+        hasWidgets = true,
+      },
+    },
+
+    ft = {
+      -- A list of patterns which will be used to protect any matching
+      -- Lean file paths from being accidentally modified (by marking the
+      -- buffer as `nomodifiable`).
+      nomodifiable = {
+        -- by default, this list includes the Lean standard libraries,
+        -- as well as files within dependency directories (e.g. `_target`)
+        -- Set this to an empty table to disable.
+      },
+    },
+
+    -- Abbreviation support
+    abbreviations = {
+      -- Enable expanding of unicode abbreviations?
+      enable = true,
+      -- additional abbreviations:
+      extra = {
+        -- Add a \wknight abbreviation to insert ♘
+        --
+        -- Note that the backslash is implied, and that you of
+        -- course may also use a snippet engine directly to do
+        -- this if so desired.
+        wknight = "♘",
+      },
+      -- Change if you don't like the backslash
+      -- (comma is a popular choice on French keyboards)
+      leader = "\\",
+    },
+
+    -- Enable suggested mappings?
+    --
+    -- false by default, true to enable
+    mappings = true,
+
+    -- Infoview support
+    infoview = {
+      -- Automatically open an infoview on entering a Lean buffer?
+      -- Should be a function that will be called anytime a new Lean file
+      -- is opened. Return true to open an infoview, otherwise false.
+      -- Setting this to `true` is the same as `function() return true end`,
+      -- i.e. autoopen for any Lean file, or setting it to `false` is the
+      -- same as `function() return false end`, i.e. never autoopen.
+      autoopen = true,
+
+      -- Set infoview windows' starting dimensions.
+      -- Windows are opened horizontally or vertically depending on spacing.
+      width = 50,
+      height = 20,
+
+      -- Put the infoview on the top or bottom when horizontal?
+      -- top | bottom
+      horizontal_position = "bottom",
+
+      -- Always open the infoview window in a separate tabpage.
+      -- Might be useful if you are using a screen reader and don't want too
+      -- many dynamic updates in the terminal at the same time.
+      -- Note that `height` and `width` will be ignored in this case.
+      separate_tab = false,
+
+      -- Show indicators for pin locations when entering an infoview window?
+      -- always | never | auto (= only when there are multiple pins)
+      indicators = "auto",
+    },
+
+    -- Progress bar support
+    progress_bars = {
+      -- Enable the progress bars?
+      enable = true,
+      -- What character should be used for the bars?
+      character = "│",
+      -- Use a different priority for the signs
+      priority = 10,
+    },
+
+    -- Redirect Lean's stderr messages somehwere (to a buffer by default)
+    stderr = {
+      enable = true,
+      -- height of the window
+      height = 5,
+      -- a callback which will be called with (multi-line) stderr output
+      -- e.g., use:
+      --   on_lines = function(lines) vim.notify(lines) end
+      -- if you want to redirect stderr to `vim.notify`.
+      -- The default implementation will redirect to a dedicated stderr
+      -- window.
+      on_lines = nil,
+    },
+  },
+}