feat!: support lazy-loading plugin configs

[mrcjkb]

This PR adds support for lazy-loading configs (which rocks-lazy.nvim will be able to use to integrate with rocks-config.nvim):

• By default, don't load configs for opt plugins (this is a BREAKING CHANGE).
  • Config bundles are still loaded eagerly.
• Add a Lua API, require("rocks-config").configure(plugin) for loading opt plugins.

Apart from that, I've updated the Nix infrastructure and added some basic integration tests.

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(installer): some installer bugfix

• [x] Pull request title has the appropriate conventional commit prefix.

If applicable:

• [x] Tested
  • [x] Tests have been added.
  • [ ] Tested manually (steps in PR description).
• [x] Updated documentation.

[mrcjkb]

~Damn, this is based off a really old state...~