Introduce enumerated attribute reflection IDL

- [ReflectEnum=identifier] can be used alongside [Reflect] on DOMString and DOMString? attributes to identify the IDL enum which holds the known value keywords.

Additionally, 3 new extended attributes are allowed on enums:

- [InvalidValueDefault] and [InvalidValueDefault=string] sets the value to be used when the content attribute has an invalid value.

- [MissingValueDefault] and [MissingValueDefault=string] sets the value to be used when the content attribute is missing.

- [EmptyValueDefault] and [EmptyValueDefault=string] sets the value to be used when the content attribute is the empty string.

Author: lukewarlow