feat(lang): add elm (#3023)

Adding https://elm-lang.org support.

- Adds `elm` Treesitter parsers
- Adds `elm-language-server` language server
- Adds `elm-format` formatter

## LSP Configuration

The default
[elm-language-server](https://github.com/elm-tooling/elm-language-server)
configuration matches all [currently supported features for Neovim
LSP](https://github.com/elm-tooling/elm-language-server#editor-support).

## A note on elm-test and elm-review

`elm-test` is not directly supported by [neo-test](../test/neotest) but
it's available via
[neotest-vim-test](https://github.com/nvim-neotest/neotest-vim-test).

However, it is common practice to peruse both
[elm-test](https://github.com/elm-explorations/test/) and
[elm-review](https://github.com/jfmengels/elm-review) directly via the
command line.

---------

Co-authored-by: gacallea <gacallea@users.noreply.github.com>
Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/lazyvim/plugins/extras/lang/elm.lua

@@ -0,0 +1,42 @@
+return {
+  recommended = {
+    ft = { "elm" },
+    root = { "elm.json" },
+  },
+
+  {
+    "nvim-treesitter/nvim-treesitter",
+    opts = function(_, opts)
+      if type(opts.ensure_installed) == "table" then
+        vim.list_extend(opts.ensure_installed, { "elm" })
+      end
+    end,
+  },
+
+  {
+    "williamboman/mason.nvim",
+    opts = function(_, opts)
+      opts.ensure_installed = opts.ensure_installed or {}
+      vim.list_extend(opts.ensure_installed, { "elm-format" })
+    end,
+  },
+
+  {
+    "stevearc/conform.nvim",
+    optional = true,
+    opts = {
+      formatters_by_ft = {
+        elm = { "elm_format" },
+      },
+    },
+  },
+
+  {
+    "neovim/nvim-lspconfig",
+    opts = {
+      servers = {
+        elmls = {},
+      },
+    },
+  },
+}