examples: gather all simple "backend adapter" example into one

[juliusmarminge]

We should think on how we want to structure ezamples. Having the same vite app like 10 times hurts CI time a looot, especially from forks that cant use the remote cache.

I wonder if a single vite app "connecting" to localhost:3000 and then all the backend adapters can be like 2 files 🤔 food for thought.

Really it could even just be a single example with a single router, and then just a bunch of different `dev:hono`, `dev:fastify` scripts that starts a different server 🤔

Originally posted by @juliusmarminge in #364 (review)

[juliusmarminge]

I'm thinking something like:

examples/
  backend-adapters/
    client/ # a vite react app that proxies /api to localhost:3000
    server/
      src/
        router.ts # a single upload router
        express.ts # an express server
        fastify.ts # a fastify server
        elysia.ts
        ...

then have scripts like dev:elysia which starts the vite app and the elysia server.

this should greatly reduce the concurrency limit for turbo which is slowing down CI significantly right now