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/constructor/default.wren

@@ -1,8 +0,0 @@
-class Foo {
-  toString { "Foo" }
-}
-
-// Classes get an argument-less "new()" by default.
-var foo = Foo.new()
-IO.print(foo is Foo) // expect: true
-IO.print(foo.toString) // expect: Foo

test/language/constructor/default_calls_new.wren

@@ -1,9 +0,0 @@
-class Foo {
-  construct new() {
-    IO.print("Foo.new()")
-  }
-}
-
-class Bar is Foo {}
-
-Bar.new() // expect: Foo.new()

test/language/constructor/no_default.wren

@@ -1,6 +1,4 @@
-class Foo {
-  construct real() {}
-}
+class Foo {}
 
-// Classes do not get an argument-less "new()" if they define a constructor.
+// Classes do not get a constructor by default.
 var foo = Foo.new() // expect runtime error: Foo metaclass does not implement 'new()'.

test/language/constructor/object.wren

@@ -1,11 +0,0 @@
-// Tests that Object implements new(). The only way to call that is through a
-// super() call in a subclass, so this does that.
-
-class Foo {
-  construct new() {
-    super() // Should not cause a no method error.
-    IO.print("ok")
-  }
-}
-
-Foo.new() // expect: ok