[heycam/webidl] Better regular expression for identifiers (#632)

See https://github.com/heycam/webidl/pull/612#pullrequestreview-197260832. Perhaps we need something that disallows `--`, but still meets the other criteria.

(This seems fairly minor since I'd expect those writing standards to keep to relatively good practice.)

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https://github.com/heycam/webidl/issues/632