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 release-please workflow that creates release PRs with SemVer versioning based on conventional commits.
• Add a workflow that publishes tags to LuaRocks when a tag or release is pushed.

The workflows are based on this guide by @vhyrro.

Notes:

• On each merge to master, the release-please workflow creates (or updates an existing) release PR.
• You decide when to merge release PRs. Doing so will result in a SemVer tag, and a GitHub release, which will trigger the luarocks workflow.
• In the luarocks github action, 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. (@rcarriga has an API key for the neotest account).
• 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.