fix(web): detect secure context for subtle crypto

[HamdaanAliQuatil]

closes #159

[jonasfj]

Minor improvement, but let's consider putting the logic into the subtle getter

[jonasfj]

Suggested change

	@JS('subtle')
	external JSSubtleCrypto? get subtleOrNull;
	@JS('subtle')
	external JSSubtleCrypto? get _subtleOrNull;
	JSSubtleCrypto get subtle => requireSubtleCrypto();

Then we don't need to refactor the rest of the code.

We can even make _requireSubtleCrypto a private function, or inline it inside the subtle getter.