Function parse_property_block 

pub fn parse_property_block(
    context: &ParserContext<'_>,
    input: &mut Parser<'_>,
    name: PropertyRuleName,
    source_location: SourceLocation,
) -> Result<PropertyRegistration, ParseError>

Parse the block inside a @property rule.

Valid @property rules result in a registered custom property, as if registerProperty() had been called with equivalent parameters.