feat: Vite environment API

[teemingc]

closes https://github.com/sveltejs/kit/issues/13692 closes https://github.com/sveltejs/kit/issues/1712 part of https://github.com/sveltejs/kit/issues/3535 closes https://github.com/sveltejs/kit/pull/13739 closes https://github.com/sveltejs/kit/pull/13357 closes https://github.com/sveltejs/kit/pull/12637

w.i.p and still has a lot of problems to solve. Needed to open this PR to test installing this from pkg.pr.new since linking locally with pnpm symlinks to the local kit repo. Then, when we try importing kit code from inside an environment, it results in a path which doesn't exist such as /packages/kit/.... Instead, it should probably import from node_modules/@sveltejs/kit/....

This PR adopts the Vite Environment API in kit and changes adapter-cloudflare to use a similar approach to Remix where the user can specify their own worker.js file. Otherwise, we can generate the wrangler config and worker for them. This approach integrates well with vite-plugin-cloudflare and provides users with direct access to the worker script, allowing them to export Durable Objects and add additional handlers.

Thanks to @jamesopstad for https://github.com/sveltejs/kit/pull/12637 and writing up the Google Doc explaining the Remix and vite-plugin-cloudflare integration. Resources include:

• https://vite.dev/guide/api-environment.html
• https://github.com/cloudflare/workers-sdk/tree/main/packages/vite-plugin-cloudflare
• https://github.com/remix-run/react-router/tree/main/playground/vite-plugin-cloudflare

TODO

• [ ] Cloudflare dev
• [ ] Cloudflare build
• [ ] Node
• [ ] vite-plugin-netlify
• [ ] Vercel?
• [ ] SPA/static?

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