ci: semver luarocks releases

[mrcjkb]

Hey :wave:

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 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 main, 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.
• 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 (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.