feat: luarocks/rocks.nvim support

[mrcjkb]

Hey 👋

Summary

This PR is part of a push to get neovim plugins on luarocks.org.

See also:

• rocks.nvim, a new luarocks-based plugin manager.
• this blog post.

Things done:

• Add a workflow that generates a rockspec for Neovim plugins and publishes tags to luarocks.org when a tag or release is pushed.

I noticed there's an existing catppuccin luarock, but it cannot be used to set the Neovim colorscheme, as it is missing the rtp paths.

• I've set the package name in the luarocks workflow to catppuccin.nvim.
• It might be a good idea to add a section to the readme, telling rocks.nvim users to use :Rocks install catppuccin.nvim and not :Rocks install catppuccin?

Notes:

Important

• For the luarocks workflow to work, someone with a luarocks.org account will have to add their API key to this repo's GitHub actions secrets.

• As you use GitHub releases to create tags (with the release-please action), you may need to add a PA token for the workflow to be triggered automatically.

• Tagged releases are installed locally and then published to luarocks.org.
• Due to a shortcoming in luarocks.org (label doesn't get picked up from rockspec luarocks/luarocks-site#188), the neovim and/or vim labels have to be added to the luarocks package manually (after the first upload), for this plugin to show up in https://luarocks.org/labels/neovim or https://luarocks.org/labels/vim, respectively.