[epic] Isograph router integration

[rbalicki2]

This is a sketch of how routing in Isograph might work.

• Certain entrypoints are associated with URLs, e.g. entrypoint Query.PetDetailPage($id: ID!) associated with pet/:id.
• At build time, a type is generated, such as:

type AnyRoute = {
  kind: 'Query.PetDetailPage',
  params: { id: number },
} | /* other routes */

creating an instance of this type is assumed to be cheap. i.e. doing so doesn't send a network request!

• Client fields have a navigateToEntrypoint function, e.g.:

function MyClientField({ data, navigateToEntrypoint, preloadEntrypoint }) {
  const onClick = () => navigateToEntrypoint({ kind: 'Query.PetDetailPage', params: { id: 123 } });
}

• The router has a generated function that turns an AnyRoute object into what we need:

function preloadRoute(route: AnyRoute, shouldFetch: ShouldFetch): RouteState {
  switch (route.kind) {
    case 'Query.PetDetailPage': {
      // pseudo code here
      const entrypointImport = import('../__isograph/Query/PetDetailPage/entrypoint.tsx')
      entrypointImport.then(value => makeNetworkRequest(value.entrypoint);
      return entrypointImport;
    }
  }
}

• Certain entrypoints can be pre-loaded (i.e. hard required) in the router if you choose
• In particular, we can probably pre-fetch: nothing, just the NetworkRequestInfo (with or without normalization Ast), the NetworkRequestInfo and the readerWithRefetchQueries, etc.

THIS WILL BE SO GOOD