fish: support theme plugins, take 2

[NovaViper]

Fixes #3724
Add change script that adds in theme files for plugins that have them.

Supersedes #7904, switched from using recursive import to onChange script, since recursive imports can cause "Import From Derivation" issues (especially when the allow-import-from-derivation setting is disabled Discuss from:
#7904 (comment)

Description

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• 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.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[NovaViper]

🤔 I'm wondering if I should also add a new news entry

[NovaViper]

cc for review: Guanran928 khaneliman

[NovaViper]

I gave it a test locally and it worked though it does leave behind stuff.. unsure how to get the lingeirng files cleaned up 🤔

[rycee]

Perhaps it would be possible to create a derivation that takes the plugins as input and produces a whole directory structure as output for use with xdg.configFile."fish/themes" = { source = mkThemeDirs cfg.plugins; recursive = true; }.

A bit similar to dbus.nix:

home-manager/modules/dbus.nix

Lines 27 to 33 in 5f06cea

	xdg.dataFile."dbus-1/services" = {
	recursive = true;
	source = pkgs.symlinkJoin {
	name = "user-dbus-services";
	paths = cfg.packages;
	stripPrefix = "/share/dbus-1/services";
	};

[NovaViper]

Perhaps it would be possible to create a derivation that takes the plugins as input and produces a whole directory structure as output for use with xdg.configFile."fish/themes" = { source = mkThemeDirs cfg.plugins; recursive = true; }.

A bit similar to dbus.nix:

home-manager/modules/dbus.nix

Lines 27 to 33 in 5f06cea

	xdg.dataFile."dbus-1/services" = {
	recursive = true;
	source = pkgs.symlinkJoin {
	name = "user-dbus-services";
	paths = cfg.packages;
	stripPrefix = "/share/dbus-1/services";
	};

🤔 Only issue with that is would it account for multiple plugins. Say if someone wants to try out some themes (or have a dark mode and light mode theme), every theme would have to go into the exact same folder. My previous PR handled that issue fine but unfortunately introduced IFD issues