New extended attribute grammars

[lukewarlow]

URL: whatwg/webidl#1503

4 new ExtendedAttribute grammars are being added for use in HTML, ExtendedAttributeString (e.g. [Reflect="foo"]), ExtendedAttributeInteger (e.g. [ReflectDefault=2]), ExtendedAttributeDecimal (e.g. [ReflectDefault=3.0] and ExtendedAttributeIntegerList (e.g. [ReflectRange=(1, 100)]).

cc: @marcoscaceres @sidvishnoi for ReSpec

[saschanaz]

I don't think it will be a breaking change.