Export PredicateType from types.ts to avoid TS4058 error in fork.ts.
Export PredicateType from types.ts to avoid TS4058 error in fork.ts.

Since defBuiltInType returns a PredicateType, the built-in types
(e.g. isString, isFunction) are all instances of PredicateType, and thus
the values of the builtInTypes object all depend on PredicateType, so the
PredicateType class needs to be exported to avoid the following error:

  [ts] Return type of exported function has or is using name
  'PredicateType' from external module
  "/Users/ben/dev/ast-types/lib/types" but cannot be named. [4058]

benjamn/ast-types,benjamn/ast-types,benjamn/ast-types