Provide visibility to running synchronously (#16)

Motivation
----------
There are cases in complex systems where it is helpful for code to know that it is running within a call to AsyncHelper.RunSync. Furthermore, retrieving the previously replaced synchronization context can also be helpful. For example, a call to Form.ShowDialog in a WinForms app may want to reinstall the WindowsFormsSynchronizationContext since a new message pump will be running to process continuations.

Modifications
-------------
- Add IsRunningSynchronously property
- Add GetReplacedSynchronizationContext method
- Track the fact we're in the call to RunSync and processing work on its calling thread using a thread static

Add null checks (#14)

Motivation
----------
Better exceptions with preconditions.

Modifications
-------------
Add a ThrowHelper for .NET < 6 and then use it or
ArgumentNullException.ThrowIfNull.

Enable trim/AOT compatibility (#10)

Add optional state parameters to AsyncHelper (PHNX-12680) (#4)

Motivation
----------
In some scenarios this can reduce heap allocations of closures.

Modifications
-------------
Add the optional state parameter and matching unit tests.

Drop .NET 5 target since it is EOL and target .NET 6.

https://centeredge.atlassian.net/browse/PHNX-12680

Ensure continuations run with sync task completion (ARCH-98) (#2)

Motivation
----------
There is currently a corner case when continuations are queued by a
synchronously completing task where those continuations are never
executed.

Modifications
-------------
When the task completes synchronously queue all remaining continuations
on the parent synchronization context.

Switch build/tests to .NET 6 and language to C# 10.

https://centeredge.atlassian.net/browse/ARCH-98

Add strong naming