Support local properties in generators

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

For example:

```pkl
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.

```pkl

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`).

Start by reviewing the frame changes in #1429 and the roles of ReadFrameSlotNode and WriteFrameSlotNode mentioned in the issue. Confirm that a local property inside a for-generator can be resolved and written to the generator member frame, then verify the Pkl example works without the nested-generator workaround.

Written by the indexing model from the issue text.