feat(vimtex): add which-key mapping descriptions

[Subjective]

Before

After

[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)

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

[Uzaaft]

@Subjective Why didn't you use the keys table in lazy.nvim to setup the keys?

[Subjective]

@Uzaaft VimTex sets up the keybinds by default upon entering a latex buffer, but doesn't add the descriptions necessary for which-key. The main reason for not using the lazy table is that I want the labels to be specific to that buffer.

[mehalter]

Cool thanks. Just be sure to track the changes on vimtex and come back when these mappings inevitably change 😅