[Pitch][SerialExecutor] Improved Custom SerialExecutor isolation checking

[ktoso (Konrad 'ktoso' Malawski 🐟🏴‍☠️)]

Another notable reason for this API from the SE-0424 review thread:

@Nickolas_Pohilets

I would like to have a Bool returning API to support isolated deinit. Implementation of the isolated deinit uses swift_task_isCurrentExecutor() to choose between fast path (no hopping) and slow path (hopping).