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/language/super/indirectly_inherited.wren

@@ -7,6 +7,8 @@ class A {
 class B is A {}
 
 class C is B {
+  construct new() {}
+
   foo {
     IO.print("C.foo")
     super.foo