Adjust the behavior and typing of forms when a model value is `undefined`.
In such cases, the form is not considered to have a field for that key of
the model, and navigation attempts should return `undefined`. The type
should also reflect the field's potential non-existence, as well as
correctly narrow the type when the field is non-null asserted.

`FieldNode`s were previously constructed with a static 'key', used for a few
purposes including `.value` operations (read/write). This meant that when a
`FieldNode` moved in an array via the tracking mechanism, it would be
resolved via the new path, but still write to the old path (a major bug).

This commit fixes the issue by introducing a proper reactive concept for the
field's current property key in the parent, which changes as the field moves
around.

… etc.

This commit should make the code much easier to follow by breaking apart
`FieldNode` into various pieces, each focused on a single concern.