Merge Stitch and Realm SDKs - Realm JS - phase 1

[RealmBot]

Phase 1:
[x] AppCredentials (except third-party auth)
[x] login (Realm.App.logIn())
[x] logout (Realm.User.logOut())
[x] current_user (Realm.App.currentUser())
[x] all_users (Realm.App.allUsers())
[x] sync
[x] switch_user (Realm.App.switchUser())
[x] remove_user (Realm.App.deleteUser())
[x] provider clients (Realm.App.UsernamePasswordProvider and Realm.App.AppKeyProvider)
[x] link_user
[x] partition_key
–
Phase 2:
See RJS-505

Details of above subtasks are described in this doc