Create "Core" module which can hold common extensions

[kgellci]

Some extensions are useful to live in a single place which can be used by multiple modules. One such extension is the String extension which was recently added:
https://github.com/kgellci/Area51/blob/4ca004468721b641ed20becb4ae9f5a899532706/Area51/Sources/Extensions/String%2BExtension.swift

I will create a Core module where we can store such extensions.