The parsing of private fields checks whether or not it is within a
class to determine if it is valid or not. However, the state.inClass
property is incorrect as it marks it as outside a class when the inner
class is closed.
This commit fixes this problem by replacing the state.inClass property
with a class nesting counter.
Given that private fields can only be used within classes, any code
using them must be in a strict mode context. As private fields cannot
be deleted, throw an early SyntaxError.
