Fold routes back into fetch-router

[mjackson]

This folds route definition helpers back into @remix-run/fetch-router under the existing routes package subpath while adding remix/routes as the canonical Remix facade import. This is part of #11384.

• Moves the route map primitives, helpers, and tests into packages/fetch-router
• Uses @remix-run/fetch-router/routes as the source for the generated remix/routes facade
• Updates direct package imports and workspace dependencies to use the fetch-router-owned subpath

import { createRouter } from 'remix/fetch-router'
import { get, route } from 'remix/routes'

let routes = route({
  home: get('/'),
})

let router = createRouter()
router.get(routes.home, () => new Response('Home'))

[github-actions]

Preview Build Available

A preview build has been created for this PR. You can install it using:

pnpm install "remix-run/remix#preview/pr-11388&path:packages/remix"

This preview build will be updated automatically as you push new commits.

[github-actions]

The preview branch preview/pr-11388 has been deleted now that this PR is merged/closed.