feat: extend lz.n with custom handlers

[BirdeeHub]

added require('lz.n').register_handler(lz.n.HandlerSpec)

added lz.n.Plugin.extras for extra spec items from custom handlers.

Added type declarations to avoid throwing warnings from adding extra items to the spec

This was necessary because require('lz.n.handler').init must call the custom handler, which cant reasonably be done from outside of the plugin.

[mrcjkb]

Thanks 🙏

Some notes (on top of my other comments):

• Please amend the commit according to the conventional commit specification.
  Otherwise, CI won't pick it up for the changelog and semver version.
• We should also expose the lz.n.loader.load() function as part of the public API
  (via the lz.n module). Handlers need to be able to call it, because it takes care of properly deregistering the plugins.
• This needs to be tested. Perhaps a very simple test handler that can be invoked from a callback.
  See the other tests in the spec/ directory for inspiration.
  I use busted for testing. Since you use Nix, you can just run busted or luarocks test from within the devShell.
  You can also run everything (linting, type checking, ...) with nix flake check -Lv.

[mrcjkb]

Checks are failing.
If you enter the devShell, it'll set up pre-commit hooks :)

[mrcjkb]

Im not good at writing tests due to lack of experience but thats next.

It's one of the most valuable skills to practise 😄

Please don't hesitate to ask for help if you get stuck. I suggest looking at lz_n_spec.lua for reference.
But the spec should go in its own register_handler_spec.lua file.

[mrcjkb]

Just a few nitpicks 😃

[mrcjkb]

LGTM 🎉 🚀