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/util/cmp.lua

@@ -1,6 +1,36 @@
 ---@class lazyvim.util.cmp
 local M = {}
 
+---@alias lazyvim.util.cmp.Action fun():boolean?
+---@type table<string, lazyvim.util.cmp.Action>
+M.actions = {
+  -- Native Snippets
+  snippet_forward = function()
+    if vim.snippet.active({ direction = 1 }) then
+      vim.schedule(function()
+        vim.snippet.jump(1)
+      end)
+      return true
+    end
+  end,
+}
+
+---@param actions string[]
+---@param fallback? string|fun()
+function M.map(actions, fallback)
+  return function()
+    for _, name in ipairs(actions) do
+      if M.actions[name] then
+        local ret = M.actions[name]()
+        if ret then
+          return true
+        end
+      end
+    end
+    return type(fallback) == "function" and fallback() or fallback
+  end
+end
+
 ---@alias Placeholder {n:number, text:string}
 
 ---@param snippet string