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/luasnip.lua

@@ -1,4 +1,8 @@
 return {
+  -- disable builtin snippet support
+  { "garymjr/nvim-snippets", enabled = false },
+
+  -- add luasnip
   {
     "L3MON4D3/LuaSnip",
     lazy = true,
@@ -12,43 +16,56 @@ return {
           require("luasnip.loaders.from_vscode").lazy_load()
         end,
       },
-      {
-        "nvim-cmp",
-        dependencies = {
-          "saadparwaiz1/cmp_luasnip",
-        },
-        opts = function(_, opts)
-          opts.snippet = {
-            expand = function(args)
-              require("luasnip").lsp_expand(args.body)
-            end,
-          }
-          table.insert(opts.sources, { name = "luasnip" })
-        end,
-      },
     },
     opts = {
       history = true,
       delete_check_events = "TextChanged",
     },
   },
+
+  -- add snippet_forward action
+  {
+    "L3MON4D3/LuaSnip",
+    opts = function()
+      LazyVim.cmp.actions.snippet_forward = function()
+        if require("luasnip").jumpable(1) then
+          require("luasnip").jump(1)
+          return true
+        end
+      end
+    end,
+  },
+
+  -- nvim-cmp integration
   {
     "nvim-cmp",
+    optional = true,
+    dependencies = { "saadparwaiz1/cmp_luasnip" },
+    opts = function(_, opts)
+      opts.snippet = {
+        expand = function(args)
+          require("luasnip").lsp_expand(args.body)
+        end,
+      }
+      table.insert(opts.sources, { name = "luasnip" })
+    end,
     -- stylua: ignore
     keys = {
-      {
-        "<tab>",
-        function()
-          return require("luasnip").jumpable(1) and "<Plug>luasnip-jump-next" or "<tab>"
-        end,
-        expr = true, silent = true, mode = "i",
-      },
       { "<tab>", function() require("luasnip").jump(1) end, mode = "s" },
       { "<s-tab>", function() require("luasnip").jump(-1) end, mode = { "i", "s" } },
     },
   },
+
+  -- blink.cmp integration
   {
-    "garymjr/nvim-snippets",
-    enabled = false,
+    "saghen/blink.cmp",
+    optional = true,
+    opts = {
+      accept = {
+        expand_snippet = function(...)
+          return require("luasnip").lsp_expand(...)
+        end,
+      },
+    },
   },
 }