Persist user preferences

[jayasuryat]

There are a bunch of user preferences to be stored, see #53.

TODO

• [x] Choose a persistence library (preferably a KMM one, see here)
• [x] Create a new module for persisting data
• [x] Do the thing (implement persistence logic)

[jayasuryat]

Chose russhwolf's multiplatform-settings library for persisting user preferences.

(A Kotlin Multiplatform library for persisting key-value data)