CSS custom properties/variables

TODO:

• Circular dependency check
• Animations
• Tests
  • Printing
    • currently it leaves spaces between all tokens so that 1px solid red doesn't become 1pxsolidred. But this also means rgba( 0 , 0 , 0 , 0 ), probably should be more selective here and e.g. special case commas and opening/closing tokens
    • Sort custom props when printing
• Address TODOs in the diff
• Allow variables in @define-color - do we need this at all?
• Error handling? Currently substituting variables at compute time discards any errors
• Currently I use modified libadwaita styles for testing (that's the bulk of the diff), should we introduce variables in Default somewhere?
  • This would be difficult without significantly changing the styles themselves
• Clean up commit history
• Docs