[ty] Improve support for object.<CURSOR> completions

This makes it work for a number of additional cases, like nested
attribute access and things like `[].<CURSOR>`.

The basic idea is that instead of selecting a covering node closest to a
leaf that contains the cursor, we walk up the tree as much as we can.
This lets us access the correct `ExprAttribute` node when performing
nested access.

Author: BurntSushi