Provide KStore initialiser for implicit storage directories

[xxfast]

Similar to kable, perhaps we can initialise the store's file implicitly to simplify the store creation for users

Docs: https://developer.android.com/topic/libraries/app-startup

[xxfast]

Hmm.. So I thought I had a nice solution for this with Androidx Startup, however it is not quite so straightforward

This is because the Context available for the Initializer doesn't give access to the same filesDir

context.filesDir           = /data/user/0/io.github.xxfast.nytimes.android/files
defaultDirectory.files     = /.local/share/io.github.xxfast.kstore

So that's a problem 🤔