Add kit module for all kit-related stuff

[ignatiusmb]

There will be a lot of repetitive stuff (like authentication for one), and these codes will likely exists in many projects using SvelteKit. Rather than creating a template and updating it every now and then, we could just have a reusable module under the /kit exports namespace and have all related stuff from there.

The first things that comes to mind is authentication handling with jwt and cookies, this will most likely be needed in a lot of apps that don't rely on third-party apps. Other stuff should come to mind when the time comes, but they should be tracked in a separate issue.