This PR adds initial support for listing all attributes of an object. It is exposed through a new `all_members` routine in `ty_extensions`, which is in turn used to test the functionality. The purpose of listing all members is for code completion. That is, given a `object.<CURSOR>`, we would like to list all available attributes on `object`.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.