feat!: Define configFiles with an array to prioritize configs

[phated]

This reworks the configFiles property to be defined as an array instead of an object. This makes it more verbose to specify multiple configFile lookups. The reason for this change is to have a defined ordering so the config overrides are applied in a guaranteed order.

This has another benefit in that we'll attempt to locate and read all configFiles and the users can do whatever they want with them.

Still TODO:

• [x] Update docs
• [x] Write tests for loading all of the config files

[phated]

@sttk I'm going to merge this so I can make the release. Let me know if you notice anything and we can fix in another PR.