fix: detect mutations within assignments expressions (alternative approach)

[Rich-Harris]

Alternative to #12411.

We use the extract_identifiers utility to find the identifiers in patterns, like these:

let a = ...; // ['a']
let { b, c: d } = ...; // ['b', 'd']

Patterns also occur in e.g. function declarations and assignment expressions. But there are two kinds of pattern — binding patterns and assignment patterns — and they're subtly different. Binding patterns can ultimately only contain identifiers (the things being declared), while assignment patterns can contain any lvalue (i.e. Identifier or MemberExpression in ESTree terminology).

The drawback of #12411 is that it lumps identifiers and member expressions together. This works, but it has a cost — it means that the bindings referenced in an assignment pattern are always marked as reassigned (rather than merely mutated) even if they're member expressions. This means that the compiler sees something like this...

function swap() {
  [array[1], array[0]] = [array[0], array[1]];
}

...and marks array as being reassigned, which means that we have to create a source for it:

-let array = $.proxy([1, 2]);
+let array = $.source($.proxy([1, 2]));

It's better if we can differentiate between these things. That's what this PR does: it expands the pattern unwrapping logic to include member expression nodes, and updates the AssignmentExpression logic to use it. The existing extract_identifiers function is kept with the same API, by calling unwrap_pattern under the hood but filtering out non-identifier nodes.

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