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/coding.lua

@@ -43,6 +43,9 @@ return {
             cmp.abort()
             fallback()
           end,
+          ["<tab>"] = function(fallback)
+            return LazyVim.cmp.map({ "snippet_forward", "ai_accept" }, fallback)()
+          end,
         }),
         sources = cmp.config.sources({
           { name = "nvim_lsp" },
@@ -72,9 +75,10 @@ return {
           end,
         },
         experimental = {
-          ghost_text = {
+          -- only show ghost text when we show ai completions
+          ghost_text = vim.g.ai_cmp and {
             hl_group = "CmpGhostText",
-          },
+          } or false,
         },
         sorting = defaults.sorting,
       }
@@ -105,17 +109,6 @@ return {
         table.insert(opts.sources, { name = "snippets" })
       end
     end,
-    init = function()
-      -- Neovim enabled snippet navigation mappings by default in v0.11
-      if vim.fn.has("nvim-0.11") == 0 then
-        vim.keymap.set({ "i", "s" }, "<Tab>", function()
-          return vim.snippet.active({ direction = 1 }) and "<cmd>lua vim.snippet.jump(1)<cr>" or "<Tab>"
-        end, { expr = true, silent = true })
-        vim.keymap.set({ "i", "s" }, "<S-Tab>", function()
-          return vim.snippet.active({ direction = -1 }) and "<cmd>lua vim.snippet.jump(-1)<cr>" or "<S-Tab>"
-        end, { expr = true, silent = true })
-      end
-    end,
   },
 
   -- auto pairs