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/method/operators.wren

@@ -15,7 +15,7 @@ class Foo {
   - { return "prefix -" }
 }
 
-var foo = Foo.new
+var foo = new Foo
 io.write(foo + "a") // expect: infix + a
 io.write(foo - "a") // expect: infix - a
 io.write(foo * "a") // expect: infix * a