[heycam/webidl] identifier needs to allow a leading hyphen (#564)

[CSSOM spec](https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_dashed_attribute) describes about identifiers with leading dashs, but they conflict with current WebIDL spec, because now we allow `/_?[A-Za-z][0-9A-Z_a-z-]*/` in identifiers.

So I'd like to update the pattern to allow a hypen as a leading character; i.e. `/[_-]?[A-Za-z][0-9A-Z_a-z-]*/` to make both specs consistent with each other.



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