zed-editor: add userThemes option

[m-saghir]

Description

Added support for local custom themes, each of which are added separately to ~/.config/zed/themes. Allows for users to update Zed's theming via nix-colors on rebuild.

See https://zed.dev/docs/extensions/themes for more details.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

cc @libewa

[m-saghir]

I have reverted changes related to formatting. As such, test failures are related to formatting. It doesn't conform to the project's guidelines.

[nyabinary]

I have reverted changes related to formatting. As such, test failures are related to formatting. It doesn't conform to the project's guidelines.

I think you should maintain the formatting it did, since that's project standard.