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/api/foreign_class.wren

@@ -5,6 +5,7 @@ class Api {
 
 // Class with a default constructor.
 foreign class Counter {
+  construct new() {}
   foreign increment(amount)
   foreign value
 }
@@ -53,7 +54,9 @@ var error = Fiber.new {
 IO.print(error) // expect: Class 'Subclass' cannot inherit from foreign class 'Point'.
 
 // Class with a finalizer.
-foreign class Resource {}
+foreign class Resource {
+  construct new() {}
+}
 
 var resources = [
   Resource.new(),