Add API to provide deletion of the user account by the app user. Both Realm Web and Realm JavaScript need to implement the API.

• [x] Upgrade core
• [x] Update BAAS
• [x] Add Realm.App.deleteUser() - Realm JavaScript
• [x] Add Realm.App.deleteUser() - Realm Web
• [x] Add tests - Realm JavaScript
• [x] Add tests - Realm Web
• [x] Add TS defs
• [x] Add API doc

Alternative: add argument to Realm.App.removeUser() to signal deletion server-side.

By late January 2022 apps can be rejected from Apple App Store is they can create an user but not delete the user