feat: luarocks support

[mrcjkb]

Hey :wave:

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:

• This PR is blocked by https://github.com/lewis6991/gitsigns.nvim/pull/964
• Tagged releases are installed locally and then published to luarocks.org.
• 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.
• Due to a shortcoming in LuaRocks (https://github.com/luarocks/luarocks-site/issues/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.