feat(ai): better completion/suggestions of AI engines (#4752)

## Description

The whole completion / snippets / AI is very tricky:
- multiple snippet engines
- native snippets on > 0.11 set their own keymaps, but not on 0.10
- multiple completion engines, like `nvim-cmp` and `blink.cmp`
- multiple ai completion engines that have a different API
- user's preference of showing ai suggestions as completion or not
- none of the ai completion engines currently set undo points, which is
bad

Solution:
- [x] added `LazyVim.cmp.actions`, where snippet engines and ai engines
can register their action.
- [x] an action returns `true` if it succeeded, or `false|nil` otherwise
- [x] in a completion engine, we then try running multiple actions and
use the fallback if needed
- [x] so `<tab>` runs `{"snippet_forward", "ai_accept", "fallback"}`
- [x] added `vim.g.ai_cmp`. When `true` we try to integrate the AI
source in the completion engine.
- [x] when `false`, `<tab>` should be used to insert the AI suggestion
- [x] when `false`, the completion engine's ghost text is disabled
- [x] luasnip support for blink (only works with blink `main`)
- [x] create undo points when accepting AI suggestions 

## Test Matrix

| completion   | snippets     | ai          | ai_cmp | tested? |
|--------------|--------------|-------------|--------|---------|
| nvim-cmp     | native       | copilot     | true   | ✅      |
| nvim-cmp     | native       | copilot     | false  | ✅      |
| nvim-cmp     | native       | codeium     | true   | ✅      |
| nvim-cmp     | native       | codeium     | false  | ✅      |
| nvim-cmp     | luasnip      | copilot     | true   | ✅      |
| nvim-cmp     | luasnip      | copilot     | false  | ✅      |
| nvim-cmp     | luasnip      | codeium     | true   | ✅      |
| nvim-cmp     | luasnip      | codeium     | false  | ✅      |
| blink.cmp    | native       | copilot     | true   | ✅      |
| blink.cmp    | native       | copilot     | false  | ✅      |
| blink.cmp    | native       | codeium     | true   | ✅      |
| blink.cmp    | native       | codeium     | false  | ✅      |
| blink.cmp    | luasnip      | copilot     | true   | ✅      |
| blink.cmp    | luasnip      | copilot     | false  | ✅      |
| blink.cmp    | luasnip      | codeium     | true   | ✅      |
| blink.cmp    | luasnip      | codeium     | false  | ✅      |


## Related Issue(s)

- [ ] Closes #4702

## Screenshots

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

## Checklist

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

lua/lazyvim/plugins/extras/coding/blink.lua

@@ -1,3 +1,10 @@
+if lazyvim_docs then
+  -- set to `true` to follow the main branch
+  -- you need to have a working rust toolchain to build the plugin
+  -- in this case.
+  vim.g.lazyvim_blink_main = false
+end
+
 return {
   {
     "hrsh7th/nvim-cmp",
@@ -5,8 +12,12 @@ return {
   },
   {
     "saghen/blink.cmp",
-    version = "*",
-    opts_extend = { "sources.completion.enabled_providers" },
+    version = not vim.g.lazyvim_blink_main and "*",
+    build = vim.g.lazyvim_blink_main and "cargo build --release",
+    opts_extend = {
+      "sources.completion.enabled_providers",
+      "sources.compat",
+    },
     dependencies = {
       "rafamadriz/friendly-snippets",
       -- add blink.compat to dependencies
@@ -35,7 +46,7 @@ return {
           auto_show = true,
         },
         ghost_text = {
-          enabled = true,
+          enabled = vim.g.ai_cmp,
         },
       },
 
@@ -45,6 +56,9 @@ return {
       -- experimental signature help support
       -- trigger = { signature_help = { enabled = true } }
       sources = {
+        -- adding any nvim-cmp sources here will enable them
+        -- with blink.compat
+        compat = {},
         completion = {
           -- remember to enable your providers here
           enabled_providers = { "lsp", "path", "snippets", "buffer" },
@@ -53,8 +67,28 @@ return {
 
       keymap = {
         preset = "enter",
+        ["<Tab>"] = {
+          LazyVim.cmp.map({ "snippet_forward", "ai_accept" }),
+          "fallback",
+        },
       },
     },
+    ---@param opts blink.cmp.Config | { sources: { compat: string[] } }
+    config = function(_, opts)
+      -- setup compat sources
+      local enabled = opts.sources.completion.enabled_providers
+      for _, source in ipairs(opts.sources.compat or {}) do
+        opts.sources.providers[source] = vim.tbl_deep_extend(
+          "force",
+          { name = source, module = "blink.compat.source" },
+          opts.sources.providers[source] or {}
+        )
+        if type(enabled) == "table" and not vim.tbl_contains(enabled, source) then
+          table.insert(enabled, source)
+        end
+      end
+      require("blink.cmp").setup(opts)
+    end,
   },
 
   -- add icons