Add ClassInstantiation (no direct RedHat equivalent)

The syntax for an ExtendedType and a ClassSpecifier is almost identical,
the only difference is that one starts with “extends” and the other with
“=>”. The grammar has one rule for this – classInstantiation – but the
representation in the RedHat AST is kind of odd (including a type from
CustomTree).
Add ExtendedType, ClassSpecifier
Add tests for ExtendedType, ClassSpecifier