Add gum_script_cancel() API that allows graceful termination of a running script from another thread.

Implementation details:

• QuickJS: Uses the engine's interrupt handler (JS_SetInterruptHandler()) to poll a per-script cancellation flag (is_cancelled) protected by a recursive mutex.
• V8: We use Isolate::TerminateExecution() directly and skip exceptions via a try-catch block.

Key changes summary:

• Add cancel vfunc to GumScriptInterface
• Protect dispose/unload state transitions with a per-script GRecMutex
• Move cancellation_mutex cleanup from destroy_context() to finalize() to prevent use-after-clear during dispose
• Auto-dispose the script if cancelled during load
• Handle interrupted/terminated exceptions in entrypoint execution
• Include 3 unit tests (run both on QJS and V8)

Limitations:

Only interrupts JavaScript execution. If a script is blocked in native code (e.g., a NativeFunction call or a blocking syscall), cancellation will take effect once control returns to the JS engine.

To quickly test only the added unit tests, do the following:
QJS:

> build/tests/gum-tests -p '/GumJS/Script/script_can_be_cancelled#QJS' -p '/GumJS/Script/script_cancel_is_idempotent#QJS' -p '/GumJS/Script/script_cancel_during_load_should_auto_dispose#QJS'

V8:

> build/tests/gum-tests -p '/GumJS/Script/script_can_be_cancelled#V8' -p '/GumJS/Script/script_cancel_is_idempotent#V8' -p '/GumJS/Script/script_cancel_during_load_should_auto_dispose#V8'