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/inherit_fields.wren

@@ -22,7 +22,7 @@ class Bar is Foo {
   }
 }
 
-var bar = Bar.new
+var bar = new Bar
 bar.foo("foo 1", "foo 2")
 bar.bar("bar 1", "bar 2")