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/core/map/to_string.wren

@@ -9,6 +9,7 @@ IO.print({1: {2: {}}}) // expect: {1: {2: {}}}
 
 // Calls toString on elements.
 class Foo {
+  construct new() {}
   toString { "Foo.toString" }
 }