refactor: add initial AppModel + Copyable interfaces

Author: EdricChan03

src/app/data/models/dao/copyable.ts

@@ -0,0 +1,8 @@
+export interface Copyable<T, CopyData> {
+  /**
+   * Create a copy of this model class, optionally with the specified `data` properties
+   * merged into the resulting model class.
+   * @param data Desired data to be merged into the copied model class.
+   */
+  copy(data?: CopyData): T;
+}

src/app/data/models/dao/model.ts

@@ -0,0 +1,16 @@
+import type { Copyable } from './copyable';
+
+/**
+ * Generic AppModel type to be used for Firestore collection/document references.
+ * @template M The `AppModel` type to be used for {@link Copyable}.
+ * @template Data Data to be returned by {@link AppModel.asRecord}.
+ * @template DataJson Data to be returned by {@link AppModel.toJSON}.
+ */
+export interface AppModel<M extends AppModel<M, Data, DataJson>, Data, DataJson>
+  extends Copyable<M, Partial<Data>> {
+  /** Converts this model class to its JSON equivalent, suitable for use in stringified data. */
+  toJSON(): DataJson;
+
+  /** Converts this model class to its JS object equivalent. */
+  asRecord(): Data;
+}