refactor: replace Foo.new constructor syntax with new Foo and simplify superclass constructor calls

Remove the `this new` constructor declaration syntax in favor of a `new` keyword that creates instances directly. Superclass constructors are now invoked via bare `super(args)` instead of `super.new(args)`, eliminating the semantic weirdness of calling a constructor on a partially-constructed instance. Update the compiler to treat `new` as a keyword token, replace the `isMethod` flag with `methodName`/`methodLength` fields, and change the VM's `NEW` opcode to pop the class from the stack before creating the instance. Add a default `new` native method on Object that returns `this`. Update all benchmarks and tests to use the new syntax.

test/inheritance/is.wren

@@ -1,9 +1,9 @@
 class A {}
 class B is A {}
 class C is B {}
-var a = A.new
-var b = B.new
-var c = C.new
+var a = new A
+var b = new B
+var c = new C
 
 io.write(a is A) // expect: true
 io.write(a is B) // expect: false