N-API: Enforce handle scope semantics more strictly

[addaleax]

The addons-napi/test_handle_scope is currently failing in debug mode because in at least one case a handle scope is opened but never closed (because it bails out on an error before it can finish). Obviously, the test case can be fixed, but we might want to give this a bit more thought:

• Do we want to only have this check active in debug mode? I’d say no.
• Should we error out (i.e. crash) if users don’t close all handle scopes they have opened? I’d say yes.

@nodejs/n-api