Introduce a simpler route declaration syntax, aimed at compatibility

[swain]

Motivation

This PR introduces a simplified syntax for declaring routes:

const router = createCompatRouter({
  introspection: undefined,
  using: new Router(),
})
  .post(
    '/items',
    {
      name: 'createItem',
      request: z.object({ id: z.string() }),
      response: z.object({ id: z.string() }),
    },
    (ctx) => {
      // implementation
    },
  )
  .get(
    '/items/:id',
    {
      name: 'getItemById',
      request: z.object({ filter: z.string() }),
      response: z.object({ id: z.string() }),
    },
    (ctx) => {
      // implementation
    },
  );