Pattern name^: value binds name, while name: value never does (except for identifiers)

[edemaine]

Fixes #1660 by no longer binding name in object pattern properties with literal values: name: "string", name: 42, or name: /regex/. The general rule is now this: if a property has a value (other than an identifier), then it doesn't get bound. This seems like a natural extension of how destructuring assignment works.

New feature name^: value binds name. This also works when checking other properties. For example, the pattern {parent^: {type: "BlockStatement", expressions}} binds parent and expression in two destructurings: {parent} = x, {expressions} = parent.

BREAKING CHANGE: Pattern matching with {name: literal} no longer binds name; use {name^: literal} to bind.

[STRd6]

Looks good! It seems natural to extend to regular destructuring at some point as well.

[edemaine]

Oh, that's a fun idea, like {x^: [y]} := z. (It already works within if.)

I wonder if there's a natural way to do it at the top level too, not just for object properties. Like I want function Comp(props) to bind props but also destructure... Something like function Comp(props^: {name}). This is related to the desire to have as in a pattern, like [a, b] as arr. I'm not sure we want to call it as because it's not a type operation. So maybe something involving ^.