Support register custom propery

[idoros]

@property at-rule registers a custom property for runtime type checking, setting default value and configuring inherit behavior. Stylable can improve DX of custom properties by keeping track of the registration of a property and providing static language services around them:

• completions for valid syntax types
  • multipliers for lists of values
  • combinators for complex syntax combinations
  • might be interesting to offer built in shorthand types ('<--background>')
• hover information to show the type, initial-value and inherit behavior
• completion for valid places (can suggest relevant properties inside a var() value or the entire var(--color) where appropriate)
• diagnostics
  • mismatch property in a position that can be identified to have a mismatched type
  • make sure initial-value is valid for the registered type
• Not sure if needed, but Stylable can offer fallback registration through CSS.registerProperty

Edit:
In addition transforming @property rule should:

• namespace the property (support global)
• export the property even when not used in any declaration

[barak007]

Actually we need to also transform the @Property definition to match the namespace of the custom property. This is actually a bug right now.