fix(js): build abstract classes and class expressions as class nodes (#158)

[DaisukeYoda]

Fixes #158.

The parser built only class_declaration as a class node. An abstract_class_declaration and a class expression fell through to the generic builder, so clone detection never offered them as class fragments.

Both forms now go through the class builder: an abstract class as NodeClass, and a class expression as NodeClassExpression. The class builder now also keeps type_parameters, so an import used only in a type parameter constraint or default is no longer reported as unused. This also applies to plain classes. The CFG builder treats a class expression like a class declaration.

Effects:

• The repro in ts: abstract classes and class expressions are missing from clone detection #158 finds 1 clone pair (0 before). On imbue-ai/latchkey clone pairs go from 203 to 204. sindresorhus/got does not change.
• An exported abstract class now has a name and the ClassDeclaration type, so it is recorded as a named export. If no module imports it, it is reported as unused, as a plain class already is.