refactor(@angular/build): modularize unit-test builder with runner-specific logic

The main `unit-test` builder is refactored to act as a dispatcher, delegating the core execution to runner-specific implementations. This change significantly improves the structure and maintainability of the testing infrastructure.

Key changes:
- A new `runners` directory is created within the `unit-test` builder to house runner-specific logic.
- All Vitest-related logic is extracted from the main builder and moved into `runners/vitest/`.
- The Karma bridge is moved into `runners/karma/`.
- A new `test-discovery.ts` file is introduced to centralize test finding logic, with a TODO to move the implementation there in the future.

Author: clydin