feat: luarocks/rocks.nvim support

[mrcjkb]

Hey 👋

Summary

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

See also:

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

Things done:

• Add a workflow that publishes tags to luarocks.org when a tag or release is pushed.

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.

• Tagged releases are installed locally and then published to luarocks.org.
  • If you push tags from a local checkout, the workflow is triggered automatically.
  • If you use GitHub releases to create tags, you may need to add a PA token for the workflow to be triggered automatically.
• Due to a shortcoming in LuaRocks (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.

See also this guide.