Generator object body scope is surprising

This snippet:

```pkl
qux = "outer"

foo {
when (true) {
local qux = "inner"
}
prop = qux
}
```

Produces:

```pkl
qux = "outer"
foo {
prop = "inner"
}
```

This is because generator object bodies are visible to the enclosing object.

Sometimes, names resolve either to a generator property, and sometimes, they resolve to an outer property. At parse time, we can't tell what these should resolve to. For example:

```pkl
qux = "outer"

foo {
when (true) {
qux = "inner"
}
prop = qux // resolves to inner qux
}

bar {
when (false) {
qux = "inner"
}
prop = qux // resolves to outer qux
}
```

It would be better if names declared inside of generator bodies are not visible from an enclosing object body.
Most code would still work if it resolved to implicit `this`.

We need to fix this before we can do https://github.com/apple/pkl/issues/1580

No files or tests are named. Start by reproducing both snippets and tracing name resolution for generator object bodies; compare the inner and outer `qux` cases. Done means declarations in generator bodies no longer leak into enclosing object bodies while the shown implicit-`this` behavior remains intact, allowing issue 1580 to proceed.

Written by the indexing model from the issue text.