This commit exposes the QuickJS module loader functionality
(JS_SetModuleLoaderFunc2) via an idiomatic Go API, allowing users
to define custom resolution and loading logic for ES6 modules.

Changes:
- Add `ModuleLoaderFunc` and `ModuleNormalizeFunc` types.
- Add `VM.SetModuleLoader()` to configure the custom hooks.
- Implement a thread-safe internal registry (`vms` map) to route
  incoming C callbacks back to the correct Go `*VM` instance via
  the `opaque` pointer.
- Add C-to-Go bridge functions (`goModuleLoader`, `goModuleNormalize`)
  to handle string conversion, QuickJS internal memory allocation
  (`js_malloc`), and proper pointer extraction.
- Add `throwReferenceError` helper to correctly format and throw
  JavaScript ReferenceErrors when module resolution fails.
- Add unit tests for successful module evaluation and error handling.