ci: add luarocks workflow

[mrcjkb]

Hey 👋

I noticed this plugin has some dependencies that are on luarocks...

Summary

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

• See this blog post, which follows up on a series of posts by @teto.
• See also: rocks.nvim, a new luarocks-based plugin manager.

Things done:

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

If you'd prefer automated releases, I can update this PR to use a release-please workflow that creates release PRs with SemVer versioning based on conventional commits,
based on this guide by @vhyrro.

Notes:

• Tagged releases are installed locally and then published to luarocks.org.
  On PRs, a test release is installed, but not published.
• 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 (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.

[rouge8]

I’m not really interested in increasing the maintenance burden.

[mrcjkb]

That's a shame to hear.

If you're not interested in maintaining tagged releases or conventional commits, would you be open to a cron job that publishes this plugin to luarocks with the commit count as the patch version, similar to how fzf-lua does it?

[rouge8]

Not really, sorry.