implement CSS property helper

[idoros]

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