[vm] Expose Dart_{CurrentIsolate,ExitIsolate,EnterIsolate}

For applications that want to have arbitrary number of isolates call
into native code that may be blocking, we expose the API functions that
allows those native threads to exit an isolate before running
long/blocking code.

Without the ability to exit/re-enter isolate, one may experience
deadlocks as we have a fixed limit on the number of concurrently
executing isolates atm.

In the longer term we may find a way to do this automatically
with low overhead, see [0]. But since those API functions are quite
stable and we already expose e.g. `Dart_{Enter,Exit}Scope`, I don't
see a reason not to expose `Dart_{Enter,Exit}Isolate`.

[0] Issue https://github.com/dart-lang/sdk/issues/51261

Issue https://github.com/dart-lang/sdk/issues/51254

TEST=ffi{,_2}/dl_api_exit_enter_isolate_test

Change-Id: I91c772ca962fddb87919663fea07939a498fa205
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/292722
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>