refactor(cmp): add optional where necessary (#4557)

## Description
Makes it easier to disable it for users who want to try other completion
engines, without it being pulled back by some Extra.

<!-- Describe the big picture of your changes to communicate to the
maintainers
  why we should accept this pull request. -->

## Related Issue(s)
None
<!--
  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
-->

## Screenshots

<!-- Add screenshots of the changes if applicable. -->

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/coding/tabnine.lua

@@ -2,6 +2,7 @@ return {
   -- Tabnine cmp source
   {
     "nvim-cmp",
+    optional = true,
     dependencies = {
       {
         "tzachar/cmp-tabnine",