treewide: use config in literalExpressions

[0xda157]

#1513 (comment)
cc @MattSturgeon

Things done

• Tested locally
• Tested in testbed
• Commit message follows commit convention
• Fits style guide
• Respects license of any existing code used

Notify maintainers

[stylix-automation]

Successfully created backport PR for release-25.05:

• [release-25.05] treewide: use config in literalExpressions #1518

[MattSturgeon]

Looks like there's one more instance, this time in a literalMD:

stylix/stylix/palette.nix

Lines 134 to 137 in 2b231cd

	defaultText = lib.literalMD ''
	Automatically selected from the background image. The default will
	throw if `stylix.image` is null.
	'';

Maybe it doesn't need the config. prefix given it is used within a sentence.

Alternatively, maybe it should use the full option name by interpolating it? "${options.stylix.image}"