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/inheritance/inherit_methods.wren

@@ -6,6 +6,7 @@ class Foo {
 }
 
 class Bar is Foo {
+  construct new() {}
   methodOnBar { IO.print("bar") }
   method(a, b) { IO.print("bar") }
   method(a, b, c, d) { IO.print("bar") }