[css-properties-values-api][editorial] CSS.registerProperty() requires inherits argument

[cdoublev]

The register a custom property algorithm is defined as taking inherits as an optional argument whereas it is passed from the PropertyDefinition dictionary, which defines it as required.

To register a custom property with name being a string, and optionally syntax being a string, inherits being a boolean, and initialValue being a string, ...

dictionary PropertyDefinition {
  required DOMString name; 
  DOMString syntax       = "*"; 
  required boolean   inherits; 
  DOMString initialValue; 
};