ci: add workflow for publishing to luarocks

[mrcjkb]

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.

Things done:

• Add a workflow that publishes tags to LuaRocks when a tag is pushed.
• Add a LuaRocks badge to the readme (assuming the existence of a TimUntersberger/neogit package).

Notes

• For the release workflow to work, someone with a Luarocks 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 label has to be added to the LuaRocks package manually (after the first upload), for this plugin to show up in https://luarocks.org/labels/neovim
• If you would prefer rolling releases without tags, this is also possible.

Adding the API key (screen shot)