Support local properties in generators

[bioball]

One pain point in Pkl is the inability to create for-generator-local properties.

For example:

foo {
  for (elem in bar) {
    local qux = doSomething(elem) // <-- error; for generators cannot have properties
    ["prop1"] = qux
    ["prop2"] = qux
  }
}

Currently one workaround is to wrap the for-generator in a another for-generator, e.g.

foo {
  for (qux in List(doSomething(elem)) {
    for (elem in bar) {
      ["prop1"] = qux
      ["prop2"] = qux
    }
  }
}

This should be doable after #1429 is merged; local properties can be written to the generator member frame, and we can resolve local properties to that exact slot (using ReadFrameSlotNode and WriteFrameSlotNode).

[bioball]

This is actually blocked on #1593