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/field/default_to_null.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   write { IO.print(_field) }
 }
 

test/language/field/multiple.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   set(a, b, c, d, e) {
     _a = a
     _b = b

test/language/field/nested_class.wren

@@ -1,9 +1,13 @@
 class Outer {
+  construct new() {}
+
   method {
     _field = "outer"
     IO.print(_field) // expect: outer
 
     class Inner {
+      construct new() {}
+
       method {
         _field = "inner"
         IO.print(_field) // expect: inner

test/language/field/use_before_set.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   write { IO.print(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }