implement CSS property helper #113
Description
Should be configurable to simplify specific CSS property operations, then accept syntax and offer:
- value validation
- mapping of value AST to sub-syntaxes (components, value-space syntaxes, and properties)
- format check - for some properties like margin the value can resolve meaningful formats (all/horizontal-vertical...)
blocked by CSS value matcher - #112
There is already committed interface, usage examples for margin & background and tests