Allow specify plasma workspace

[wrvsrx]

This PR allows specify plasma workspace in kde module.

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• 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

[trueNAHO]

CC: @lordkekz, @rkuklik

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@lordkekz and @rkuklik, our KDE module would not have been possible without your hard work. Do you mind adding yourself as a KDE module maintainer? The new /.github/workflows/request-reviewers.yml action will automatically notify you about PRs touching the /modules/kde/ directory. The following documentation extract explains how to add yourself as a module maintainer:

stylix/doc/src/modules.md

Lines 228 to 264 in 8d00829

	### Maintainers
	New modules must have at least one maintainer.
	If you are not already listed in the Nixpkgs `/maintainers/maintainer-list.nix`
	maintainer list, add yourself to `/stylix/maintainers.nix`.
	Add yourself as a maintainer in one of the following ways, depending on the
	number of maintainers:
	- ```nix
	{ lib, ... }:
	{
	maintainers = [ lib.maintainers.danth ];
	}
	```
	- ```nix
	{ lib, ... }:
	{
	maintainers = with lib.maintainers; [
	awwpotato
	danth
	naho
	];
	}
	```
	The main responsibility of module maintainers is to update and fix their
	modules.
	> [!NOTE]
	> If this is the first time you're adding yourself as a maintainer in Stylix,
	> the `/generated/all-maintainers.nix` file will need to be updated by running
	> `nix run .#all-maintainers`
	> ([pre-commit](./development_environment.md#pre-commit) will also automatically
	> regenerate it).