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.
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