JoinableTaskSynchronizationContext should override CreateCopy()

Bug description

The default implementation on SynchronizationContext.CreateCopy() returns a new instance of its own type.
But that type has very different behavior than JoinableTaskSynchronizationContext, so it would be incorrect to return the base type from our derived type.

Repro steps

I don't know what failures this may lead to yet. But given when ExecutionContext is captured it can (sometimes?) flow SynchronizationContext, I suspect if we can isolate when that happens we can find a repro for a bug.

Expected behavior

We should override the CreateCopy() method and have it return a copy of the current instance. The copy should behave exactly like the original, including common values for all fields so that they share queues, etc. But it should be a unique instance (not this) to avoid exposing bugs in others' code such as that learned by WPF, as pointed out by @weltkante in #347.

Additional context

@jviau noticed some test failures while working on the ReentrantSemaphore in certain conditions that he attributed to our CreateCopy() behavior, IIRC.

Start with JoinableTaskSynchronizationContext and the SynchronizationContext.CreateCopy() contract described in the issue. Trace how the current instance stores queues and other shared values, then verify that a distinct copy preserves the original behavior and shared state without returning the same object. Use the existing ReentrantSemaphore-related tests or failures, if present, to confirm the behavior.