Migrate the middleware implementation to Express.js

FWIW I don't know why `base`. The middleware code seems too compact and difficult to follow. We've wondered if we could rework it with express somehow.

_Originally posted by @johnjbarton in https://github.com/karma-runner/karma/issues/1607#issuecomment-734016574_

No file, test, or entry point is named. Start by locating the current middleware implementation and the `base` reference, then inspect how the middleware is wired before evaluating an Express.js migration. Done means the middleware has been reworked around Express.js, with the existing behavior accounted for.

Written by the indexing model from the issue text.