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/field/use_before_set.wren

@@ -3,7 +3,7 @@ class Foo {
   init { _field = "value" }  // ...before an assignment to it.
 }
 
-var foo = Foo.new
+var foo = new Foo
 // But invoke them in the right order.
 foo.init
 foo.write // expect: value