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/this/nested_class.wren

@@ -1,4 +1,6 @@
 class Outer {
+  construct new() {}
+
   method {
     IO.print(this) // expect: Outer
 
@@ -6,6 +8,8 @@ class Outer {
       IO.print(this) // expect: Outer
 
       class Inner {
+        construct new() {}
+
         method {
           IO.print(this) // expect: Inner
         }