Add Nullable type

[fernandortec]

Hey, I was working on this project and I was really tired of doing Type.Optional(Type.Union([Type.String(), Type.Null()])), just to make a nullable type, so I came up with a nullable type-safe function.
Does it make sense to implement this as a new type?

`
function Nullable(
property: T,
options?: TypeBox.ObjectOptions,
): TypeBox.TOptional<TypeBox.TUnion<[T, TypeBox.TNull]>> {
return TypeBox.Optional(
TypeBox.Union([property, TypeBox.Null()], { ...options }),
);
}
interface TWithNullable extends Omit<typeof TypeBox, ""> {
Nullable: typeof Nullable;
}

export const t: TWithNullable = {
...TypeBox,
Nullable,
};

`