🚀 Feature Request: Env var mocking for Pages

[WalshyDev]

Describe the solution

We should really add CF_PAGES and other env vars to pages dev (and if we add a build step)

this allows for a more realistic local dev and also helps for detections (like for SvelteKit'a auto adapter)

[sidharthachatterjee]

That does make sense. Is CF_PAGES relevant only at build time or runtime?

[Ekwuno]

@sidharthachatterjee mostly to build. eg: Adding CF_PAGES to the build command in SvelteKit will allow for using direct uploads with SvelteKit adapter-auto. Without it there is the added step of installing the adapter-cloudflare