feat(mobx): start work on migrations

src/mobx/interfaces/Migrate.ts

@@ -0,0 +1,11 @@
+import Store from "./Store";
+
+/**
+ * A data store which is migrated forwards.
+ */
+export default interface Migrate<K extends string> extends Store {
+    /**
+     * Migrate this data store.
+     */
+    migrate(key: K, data: Record<string, unknown>, rev: number): void;
+}