feat: luarocks/rocks.nvim support

[mrcjkb]

Hey 👋

Summary

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

See also:

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

With luarocks/rocks.nvim, it is the plugin authors' responsibility to declare dependencies - not the user's.
Installing this plugin becomes as simple as :Rocks install harpoon.

Things done:

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

The workflows are based on this guide

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.

• On each merge to master (if this lands on 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, a changelog update, and a GitHub release, which will trigger the luarocks workflow.
  If you would like versioning to start from a specific version (e.g. 2.0.0), you can configure this with a manifest.
• 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.org (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.