feat(completion): added avante-nvim plugin

[vonhyou]

📑 Description

Add a new plugin for a cursor-like experience, supports Claude, OpenAI, Copilot etc.

Closes #1168

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).

[luxus]

https://github.com/yetone/avante.nvim/wiki#keymaps-and-api-i-guess
keymaps should be done with astrocore i guess
and the plugin should load on avante commands and on entering visual mode

[kaka-ruto]

Thanks @vonhyou !

[mehalter]

This looks great! One last thing is we can add some extra stuff to help configure optional plugins :)