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/setter/instance.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   bar=(value) {
     IO.print(value)
   }

test/language/setter/result.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   bar=(value) { "result" }
 }
 

test/language/setter/same_name_as_method.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   bar=(value) { IO.print("set") }
   bar { IO.print("get") }
 }