Declare storage path constants

[kirillt]

This repo must be the "single source of truth" for the whole ARK project.

We have also ArkLib Android repo providing bindings to Rust code for Android and also some other functionality. That extra functionality will go to this repo eventually.

An important file from the Android repo defines storage path constants:

object ArkFiles {
    const val ARK_FOLDER = ".ark"
    const val STATS_FOLDER = "stats"
    const val FAVORITES_FILE = "favorites"

    // User-defined data
    const val TAG_STORAGE_FILE = "user/tags"
    const val SCORE_STORAGE_FILE = "user/scores"
    const val PROPERTIES_STORAGE_FOLDER = "user/properties"

    // Generated data
    const val METADATA_STORAGE_FOLDER = "cache/metadata"
    const val PREVIEWS_STORAGE_FOLDER = "cache/previews"
    const val THUMBNAILS_STORAGE_FOLDER = "cache/thumbnails"
}

These constants must be moved to this repo and be imported to the Android repo via bindings.