[build-utils] Add support for writing routes-manifest.json

[styfle]

• Related https://github.com/vercel/runtimes/issues/281

[codecov]

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[styfle]

@AndyBitz Does the page need to strip /index here?

[AndyBitz]

Good question. I think it doesn't matter, because /index and / should be equal

[AndyBitz]

Ah, I think it'd be best to remove it, as I just checked what Next.js does: 

  "dynamicRoutes": [
    {
      "page": "/[nice]",
      "regex": "^/([^/]+?)(?:/)?$",
      "routeKeys": {
        "nice": "nice"
      },
      "namedRegex": "^/(?<nice>[^/]+?)(?:/)?$"
    }
  ]

 for a pages/[nice]/index.js

[AndyBitz]

Not relevant because Next.js will not create an index file for it, it'll make [nice] the file itself, so yea, we can leave it as it is.

[styfle]

Currently, zero config routes have a handle: miss phase thats used to add a rewrite so that both the extension and the extensionless api file work (unless cleanUrls: true which will use a redirect instead).

However, the current routes-manifest.json doesn't support handle: miss routes, so we'll need to provide a new prop in the manifest if we want to keep the same behavior.