Fetch schemes from base16-project, reorganize lib

[Misterio77]

This is a major change, but the only breaking change is the lib function namespace.

You can now use either nix-colors-style schemes, or base16 YAML schemes directly, using the new schemeFromYAML function.

Opinionated functions have been moved into lib-contrib.

base16 has moved to an org, so with that in mind we no longer need to vendor schemes here.

Docs has been updated, and the repo was cleaned up from now-unneeded stuff.

All occurences of "color(S|s)cheme(s)" have been normalized to "colorScheme(s)", with "colorscheme(s)" as aliases.

Changelog:

• Add schemeFromYAML and schemeToYAML functions.
  • These are pure nix, and allow us to finally use base16-project's .yaml schemes without requiring users to pass pkgs or specify system.
• Use base16-project scheme repository. There is a scheme monorepo now, which makes our lives a lot easier
  • We no longer vendor schemes.
  • The schemes we previously had were upstreamed on https://github.com/base16-project/base16-schemes/pull/1
• Move opinionated functions into lib-contrib
• Update docs
• Clean up repo