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/nonlocal/mutual_recursion.wren

@@ -1,8 +1,10 @@
 class Foo {
+  construct new() {}
   static bar { Bar.new() }
 }
 
 class Bar {
+  construct new() {}
   static foo { Foo.new() }
 }