fix: allow $defs and fix circular pointers issue with $ref in root

[KurtGokhan]

Fixes #382, #370, #356

I think all of the issues above are caused by the same root cause, that $ref in root object isn't correctly resolved.

Changes in this PR:

• Removed the short-circuit in Pointer.resolve. As I see, it was there to prevent a circular reference. But it causes incorrect behavior. The root of the problem that should be fixed is that Pointer doesn't have a mechanism to detect circular (self-referencing) refs.
  • There is no test case for failing circular refs. But it's a configuration error and cannot be dereferenced anyway. I think it's an advanced use case that doesn't need to be handled.
  • The change in Pointer caused one test to break, which is circular-external-direct. At first it seems like a legit test-case but it's actually not. Changed the test case to be more meaningful.
• Added handling for $defs in addition to definitions
• Used Vitest snapshots for tests. Let me know if there is a reason to keep using the utils other tests are using.
• Ran prettier format