[heycam/webidl] Extended attributes for arguments? (#691)

```
Argument ::
    ExtendedAttributeList ArgumentRest

ArgumentRest ::
    optional TypeWithExtendedAttributes ArgumentName Default
    Type Ellipsis ArgumentName

ArgumentName ::
    ArgumentNameKeyword
    identifier
```

Currently the syntactic item `Argument` is defined as above, syntactically allowing double extended attributes only for optional arguments but not for non-optional one: `[ExtAttr] optional [Clamp] short argname`. Is this intended?

I think this causes ambiguity before reading the token `optional`, where the first extended attribute typically is for the type but not when optional.

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