Allow binding patterns in rest binding elements

[DanielRosenwasser]

See #2519 (comment).

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New grammar rules:

    BindingRestElement_[Yield]:
        ...BindingIdentifier[?Yield]
        ...BindingPattern_[?Yield]

Relevant section:

https://tc39.github.io/ecma262/#sec-destructuring-binding-patterns

Old work for disalllowing the construct was performed here: #2754.

[JsonFreeman]

Remember to test this with ObjectBindingPattern as well, not just ArrayBindingPattern

[ajafff]

@DanielRosenwasser there are actually two separate parts to this issue:

• ParameterDeclaration and VariableDeclaration
  • only needs to remove the grammar check
  • downlevel emit already works as expected
• allow BindingPattern for FunctionRestParameter
  • needs to remove the grammar check
  • currently there is no downlevel emit at all

[weswigham]

Fixed by #26017