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/arity.wren

@@ -18,7 +18,7 @@ class Foo {
   method(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) { return a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p}
 }
 
-var foo = Foo.new
+var foo = new Foo
 io.write(foo.method) // expect: 0
 io.write(foo.method(1)) // expect: 1
 io.write(foo.method(1, 2)) // expect: 3