Extend assetPrefix for global prefixing

[zvictor]

It has been great to have assetPrefix in our app. With few modifications we are able to serve nextjs next to a meteor app, served in the same domain but with a specific prefix:

mydomain.com/next/* --> nextjs
mydomain.com/* --> old meteor

The problem

In the scenario described above, we make use of assetPrefix not only for assets but for the whole application. As consequence:

1. It feels hacky to call it assetPrefix when we are actually prefixing everything.
2. As it was intended, the helpers from next/asset are only useful for proper assets. Anything outside of /static that needs to be prefixed can't be done this way.
3. The assetPrefix is not accessible from next/config. If you want to reuse assetPrefix e.g. when prefixing your routes, you can't.

Desired behaviour

↳ Option 1 (quick and dirty):

give access to assetPrefix, allowing devs to keep misusing it as a global prefix.

import { getAssetPrefix } from 'next/asset'

myRouter.setPrefix(getAssetPrefix())

↳ Option 2:

create a new prefix to cover the use case I described.

import { globalPrefix } from 'next/config'

myRouter.setPrefix(globalPrefix)

...

express().use(globalPrefix, myRouter)

assetPrefix would still exist, but globalPrefix would be its default value.

[tlenex]

Struggling with this too, seems like we have to make a custom server with all routes just for server route prefix support.

EDIT:
#3332 seems like a solution, but we need to get this merged

[timneutkens]

Tracking this here: #4998