Given that it refers to an identifier, it's somewhat confusing to have
to use .name.name to get the actual string. The type declared within
types.js is also incorrect, which this commit fixes.
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.
