Step 2: Introduce palette pipeline, generators, and multi-scheme support (base16/base24/semantic)

[make-42]

The code for this PR is written off the tip of this PR: #2289

This PR replaces the base16Scheme-centric pipeline with a more general palette system supporting:

• base16 (required)
• base24
• semantic (Material You–style)

Changes

• Add stylix/generators.nix:
  introduces stylix.palette.generators.{base16, base24, semantic}
  includes tinty + matugen implementations
• Add stylix/mappings.nix:
  introduces mappingFunction
  includes initial semantic2base16 mapping
• Replace stylix.base16Scheme with:
  stylix.palette.manual.{base16, base24, semantic}
• Add the following pipeline:
  manual/generator -> merge -> mappingFunction -> override -> base16 consumption (this will later be changed to allow for themes to be written for different schemes and user preferences making it possible to choose what schemes are consumed in order of priority for available schemes by theme module)
• override now applies after mapping (base16 only)

There probably needs to be documentation written for this...

This is a direct solution to step 2 outlined in the matugen PR.

It's still lacking how to handle consumption of non base16 schemes but I'm waiting on your input for that.

Here's what the testbeds that are now using matugen (semantic) then mapped to base16 look like:

• I certify that I have the right to submit this contribution under the MIT license
• Commit messages adhere to Stylix commit conventions
• Theming changes adhere to the Stylix style guide
• Changes have been tested locally
• Changes have been tested in testbeds
• Each commit in this PR is suitable for backport to the current stable branch

[make-42]

alright, thanks @philocalyst for your comments! I fixed it up!

[make-42]

Btw this is now a step forward compared to the code from the matugen PR feature wise

[make-42]

with this being valid usage:

but also this since matugen now provides a base16 output format:

[make-42]

with the first config behaving exactly like the code from the matugen PR

[make-42]

alright, i took what you said into account and added extra documentation to configuration.md