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.

example/import-module/cthulu.wren

@@ -1,3 +1,4 @@
 class Cthulu {
+  construct new() {}
   message { "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" }
 }

example/import-module/lovecraft.wren

@@ -1,6 +1,7 @@
 import "cthulu" for Cthulu
 
 class Lovecraft {
+  construct new() {}
   say() { Cthulu.new().message }
 }