Allow `TypeChecker` to be extended.

Currently, TypeChecker has a private constructor that prevents outside extension, but does not prevent implementing.

This results in a strange state where anyone wanting their own custom type checkers don't get the convenience of only implementing `isExactly` and must implement the entire interface.

Start by locating the TypeChecker definition and inspecting its private constructor and interface. Confirm how an external custom checker currently has to implement the full interface, then make the extension path support implementing only isExactly and verify that existing TypeChecker behavior remains intact.

Written by the indexing model from the issue text.