fix: remove default constructors and require explicit construct new() for all user classes

The compiler no longer auto-generates a default `new()` constructor for classes. All classes must now explicitly define `construct new() {}` to be instantiable. This change removes the `createDefaultConstructor` function from the compiler, eliminates the `return_this` primitive used by Object's default initializer, and adds explicit constructors to all test and example classes. Core metaclasses (Bool, Class, Null, Num, Object, Range, Sequence) now correctly reject `new()` calls with runtime errors.

test/core/sequence/is_empty.wren

@@ -2,6 +2,7 @@ IO.print([].isEmpty) // expect: true
 IO.print([1].isEmpty) // expect: false
 
 class InfiniteSequence is Sequence {
+  construct new() {}
   iterate(iterator) { true }
   iteratorValue(iterator) { iterator }
 }