feat: rocks.nvim/luarocks support

[mrcjkb]

Closes #14.

Things done:

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

• Add an scm rockspec that can be used to test luarocks installations locally, with luarocks make, and to optionally upload dev releases to luarocks with luarocks upload --api-key <API_KEY> fugit2.scm-1.rockspec

  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.

• 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.

[!TIP]

• I recommend the release-please GitHub action to automatically generate release PRs based on conventional commits.
• See also this guide.

• When installing with luarocks, it will

  • Install all of the lua dependencies automatically.
  • Check for the presence of a libgit2 library, and fail if it is not found.

• 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.

[SuperBo]

@mrcjkb, some error with rockspec

Error: Could not load rockspec file /home/runner/work/fugit2.nvim/fugit2.nvim/fugit2.nvim-0.0.0-2.rockspec (Error running file: [string "/home/runner/work/fugit2.nvim/fugit2.nvim/f..."]:5: attempt to concatenate global 'specrev' (a nil value))
stack traceback:
    [C]: in function 'on_failure'
    ...-luarocks-tag-release-scm-1/share/lua/5.1/ltr/os.lua:48: in function 'execute'
    ...release-scm-1/share/lua/5.1/luarocks-tag-release.lua:77: in function 'create_rockspec'
    ...release-scm-1/share/lua/5.1/luarocks-tag-release.lua:153: in function 'luarocks_tag_release'
    ...ag-release/scm-1/bin/luarocks-tag-release-action.lua:84: in main chunk
    [C]: ?