Features flags for ISR'd pages using SvelteKit

This repository shows how to manually implement feature flags on Vercel for pages that are ISR'd using SvelteKit.

Flow:

1. Request comes to middleware.ts. A flag cookie is looked up to see what value it is. For first time visitors the value is computed randomly.
2. Based on the cookie value and the URL a rewrite is determined (in this example, / goes either to /home-a or /home-b)
3. Request comes to ISR'd SvelteKit page, serving either /home-a or /home-b
4. Flag cookie is sent along with the response (it's set in middleware.ts, so ISR/SvelteKit itself never sees it, which is important), so revisiting user gets the same flag value
5. On client side navigation changes, SvelteKit's reroute hook in hooks.ts is invoked, also reading from that cookie to determine where to go to when navigating to /