feat: add vimtex integration

[Vinzent03]

This pr adds an integration for the vimtex plugin. This adds quite more keymaps than the other integrations, and it's more complicated to simulate the original key press, because it does not provide a lua interface.
So I'm curious what you think about this integration.

For me, a nice addition with this integration is a better description in which-key, because it now shows the actual action and not just vimtex-]]).

[mawkler]

Thank you for this PR. And thank you for providing great documentation!

Could you also add these two things to the README.md please?

1. Update the configuration table in the default options so that it includes the vimtex options
2. Add a Vimtex section to Usage with a table with mappings and explanations

Could you also make sure that you use 's instead of " for your strings for formatting consistency?

[Vinzent03]

Great, all your requested changes should be implemented.
Additionally, I've fixed the keymap for comment end and changed the mode to normal only, because I couldn't get it to work in operator pending mode.

[mawkler]

Awesome!

Yeah that's odd that the operator pending-mapping doesn't work, I tried it as well. Perhaps we should keep the visual mode mapping as well ('x')? That one seems to work

[Vinzent03]

Oh yeah you are right. I've added it back in. I've also added a note to the readme.

[mawkler]

Great work! Thank you very much for your contribution!