Extracted from https://github.com/vercel/next.js/pull/13333, the same exact code lives in that PR as well, but we can merge this separately if it makes reviewing https://github.com/vercel/next.js/pull/13333 easier
This PR does 3 things
- deduplicate code from build and next-dev-server that loads custom routes from next.config.js (`loadCustomRoutes`)
- in `loadCustomRoutes`, load these rewrites, headers and redirects configs concurrently instead of sequentially.
- in next-server, make `this.customRoutes` always defined, this allows us to remove the big `if` around its initialization code in `generateRoutes`, which in turn makes it possible to reuse this code for other routing than user defined routes, which is how https://github.com/vercel/next.js/pull/13333 adds its redirects.
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