Support explicit cancellation of Wasm execution

[jtakakura]

Summary

This PR implements a mechanism to explicitly cancel a running WebAssembly instance from an external thread, as requested in #27.

Previously, execution could only be stopped via pre-set fuel limits or deadline timeouts. This change introduces a thread-safe cancelled flag in the Vm struct, allowing host applications to interrupt long-running or infinite loops from a different thread.

Proposed Changes

Core VM Implementation

• Added cancelled: std.atomic.Value(bool) to the Vm struct for thread-safe concurrent access.
• Implemented Vm.cancel() to set the flag and ensured Vm.reset() clears it at the start of each invocation.
• Integrated the check into consumeInstructionBudget(). This allows the VM to stop execution at the next checkpoint (~every 1024 instructions).
• Updated the JIT execution path (jitFuelCheckHelper) to honor the cancellation flag.

Public API

• Zig API: Added WasmModule.cancel() as a high-level method to request cancellation.
• C API: Added zwasm_module_cancel(zwasm_module_t *module) to include/zwasm.h and src/c_api.zig.
• Errors: Added error.Canceled and updated the CLI to provide a descriptive error message when cancellation occurs.

Testing

• Added unit tests in src/vm.zig that simulate a background thread canceling both interpreter and JIT-compiled loops.
• Enhanced C API tests in test/c_api/test_ffi.c to verify that the zwasm_module_cancel symbol is correctly exported and functional in a multi-threaded C environment.

Related Issue

Closes #27