This fixes the problem that `SynchronizationContext.Current` would be null
during the call to `_EnterTree`, `_Ready` and the first call to `_Process` thus
the task continuations would be scheduled outside the main thread, which is unexpected and might lead to crashes.
With this change, task continuations are scheduled always on the main thread and so async/await can be used without any explicit synchronization, which is what is expected.
Fixes#18849
(cherry picked from commit f25240cfe6)
