Integrate with WebAssembly multithreading

[SteveSandersonMS]

As multithreading is added to the core .NET WebAssembly runtime, we need to ensure this doesn't lead to invalid/dangerous scenarios for Blazor components.

• Min bar: if the renderer is accessed from the wrong thread, throw
• Ideal: sync context with semantics equivalent to Blazor Server or WebView, so InvokeAsync works

This is going to come in late so we should take a risk-averse position on how much we attempt to implement here.

[pavelsavara]

Related dotnet/runtime#86759