[heycam/webidl] Consider renaming all legacy extended attributes (#350)

I'm not sure this is worth the churn, but if we wanted to, let's do it all at once.

- LenientSetter -> LegacyLenientSetter
- LenientThis -> LegacyLenientThis
- NamedConstructor -> LegacyNamedConstructor
- OverrideBuiltins -> LegacyOverrideBuiltins??
- Replaceable -> LegacyReplaceable?
- TreatNonObjectAsNull -> LegacyTreatNonObjectAsNull
- TreatNullAs=EmptyString -> LegacyNullToEmptyString (different naming scheme than LegacyTreatNonObjectAsNull since it operates on types, not callback functions)
- Unforgeable -> LegacyUnforgeable???

I have attached varying degrees of ?s for the ones I am unsure on.

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https://github.com/heycam/webidl/issues/350