v0.13.3

• add experimental JavaScript backend support. Features include:
  • all IO-independent abstractions are available, including task group, timeout, async queue, semaphore etc.
  • a new package, moonbitlang/async/js_async, provides integration with JavaScript promise:
    • wait for JavaScript promise, with automatic cancellation support using AbortController
    • export async MoonBit function as JavaScript promise, again with native cancellation support via AbortSignal
• add @async.is_being_cancelled for detecting whether current task is being cancelled. This is useful for avoiding dead loop for some code patterns, such as writing catch-all error handler inside loop body
• ignore SIGPIPE, so that writing to a closed pipe result in a normal error instead of crashing the program
• some other bugfix