Add serve-index as connect middleware for @parcel/reporter-dev-server

[kai-dorschner-twinsity]

Use Case
When loading parcel in dev mode with multiple entry points (parcel serve ./src/*.html --open ) it would be nice to see a directory-listing and choose from there what file you want to see.

If there is more people than just me I can possibly produce a draft plugin / PR for the existing dev server.

Vision
It could also be useful to have an API to add any middleware to the dev server, not just this in particular.

Ref: https://www.npmjs.com/package/serve-index