fix: make async runtime and context Send/Sync without parallel feature

[richarddd]

Issue

Fixes #495

Description of changes

AsyncRuntime, AsyncContext, and WithFuture are now unconditionally Send/Sync so async_with can be used with multi-threaded executors without requiring the parallel feature, since the underlying Arc<async_lock::Mutex<_>> is already thread-safe.

Checklist

• Added change to the changelog
• Created unit tests for my feature if needed

[Sytten]

I am very unconvinced about that one. Parallel also enables a bunch of things like Mutex for Ref, JS_UpdateStackTop calls, etc. that are required for it to work correctly multi-threaded.

[richarddd]

Yeah true, let's ditch it and close the issue. Better to be safe here