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/ai/codeium.lua

@@ -1,18 +1,42 @@
 return {
 
+  -- codeium
+  {
+    "Exafunction/codeium.nvim",
+    cmd = "Codeium",
+    build = ":Codeium Auth",
+    opts = {
+      enable_cmp_source = vim.g.ai_cmp,
+      virtual_text = {
+        enabled = not vim.g.ai_cmp,
+        key_bindings = {
+          accept = false, -- handled by nvim-cmp / blink.cmp
+          next = "<M-]>",
+          prev = "<M-[>",
+        },
+      },
+    },
+  },
+
+  -- add ai_accept action
+  {
+    "Exafunction/codeium.nvim",
+    opts = function()
+      LazyVim.cmp.actions.ai_accept = function()
+        if require("codeium.virtual_text").get_current_completion_item() then
+          LazyVim.create_undo()
+          vim.api.nvim_input(require("codeium.virtual_text").accept())
+          return true
+        end
+      end
+    end,
+  },
+
   -- codeium cmp source
   {
     "nvim-cmp",
-    dependencies = {
-      -- codeium
-      {
-        "Exafunction/codeium.nvim",
-        cmd = "Codeium",
-        build = ":Codeium Auth",
-        opts = {},
-      },
-    },
-    ---@param opts cmp.ConfigSchema
+    optional = true,
+    dependencies = { "codeium.nvim" },
     opts = function(_, opts)
       table.insert(opts.sources, 1, {
         name = "codeium",
@@ -30,4 +54,18 @@ return {
       table.insert(opts.sections.lualine_x, 2, LazyVim.lualine.cmp_source("codeium"))
     end,
   },
+
+  {
+    "saghen/blink.cmp",
+    optional = true,
+    opts = {
+      sources = {
+        compat = vim.g.ai_cmp and { "codeium" } or nil,
+      },
+    },
+    dependencies = {
+      "codeium.nvim",
+      vim.g.ai_cmp and "saghen/blink.compat" or nil,
+    },
+  },
 }