refactor: improve device interface types (#996)

app/assets/javascripts/Device/WebOrDesktopDeviceInterface.ts

@@ -0,0 +1,9 @@
+import { DeviceInterface, RawKeychainValue } from '@standardnotes/snjs'
+
+export interface WebOrDesktopDeviceInterface extends DeviceInterface {
+  readonly appVersion: string
+
+  getKeychainValue(): Promise<RawKeychainValue>
+
+  setKeychainValue(value: RawKeychainValue): Promise<void>
+}