`playlists` key should be `collections`

[danpastori]

Discussed in #510

^{Originally posted by danpastori May 4, 2022}

👉 Describe the problem

When working with podcasts and other audio groupings, the key playlists doesn't make much sense. We should refactor this to collections to accommodate more audio solutions.

👥 Problem evidence & reach

Anyone using AmplitudeJS for anything but songs.

🥰 Describe the "impact" on users?

Their code will match what they are using AmplitudeJS for.

🏆 How to solve this problem

This is a breaking change so we need to thoroughly document this. We rename and refactor the playlists key to be collections and allow users to define audio and other metadata for the grouping that we can use for display.

💯 How do we validate the problem is solved?

The user can submit a collections key to their initialize function that groups audio files.