fix: better binding interop between runes/non-runes components

[dummdidumm]

Ensure binding from legacy component passed to runes component updates correctly. This is done by also using the prop(..) variant for a property if it's mutated in runes mode, and then figuring out at runtime whether or not the parent should be notified or not

fixes #12032

Also:

• fixes an adjacent bug I came across: console.log(foo.bar = true) did not log true
• fixes an adjacent bug where rest on let { foo, ...rest } = $props() contained private properties like $$slots etc
• refactored the knowledge of "is this a prop(..) prop" into a common function

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[Rich-Harris]

IIUC in the mutation case we're always calling the prop function with the value of calling itself:

stuff(stuff().blah += 1, stuff());

Given that the prop already knows its own value, would it be simpler to do this?

stuff(stuff().blah += 1, true);

Then the function returned from prop would be like this:

return function (/** @type {any} */ value, mutation) {
  if (arguments.length > 0) {
    // We don't want to notify if the value was mutated and the parent is in runes mode.
    // In that case the state proxy (if it exists) should take care of the notification.
    // If the parent is not in runes mode, we need to notify on mutation, too, that the prop
    // has changed because the parent will not be able to detect the change otherwise.
    if (!runes || !mutation || legacy_parent) {
      // upon reassignment only the first argument is used
      /** @type {Function} */ (setter)(mutation ? getter() : value);
    }
    return value;
  }
  
  return getter();
};

[dummdidumm]

good catch, simplified