Detect unused Xcode asset catalog colors

[ileitch]

Custom colors created in an Xcode asset catalog are not detected as unused. They are declared within the file /Build/Intermediates.noindex/Project.build/Debug/Project.build/DerivedSources/GeneratedAssetSymbols.swift, which is currently excluded by the default indexExclude pattern **/*?.build/**/*.

Simply allowing this file to be scanned isn't a suitable approach, as it includes other declarations that may also be correctly identified as unused, but due to its generated nature, cannot be removed individually.

To resolve this, we likely need a new mechanism design for detecting unused generated asset catalog declarations.