feat: add basic pasters (#7331)

* feat: implement basic IPaster interface

* feat: add pasters for blocks and workspace comments

core/interfaces/i_paster.ts

@@ -0,0 +1,23 @@
+/**
+ * @license
+ * Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {Coordinate} from '../utils/coordinate.js';
+import {WorkspaceSvg} from '../workspace_svg.js';
+import {ICopyable, CopyData} from './i_copyable.js';
+
+/** An object that can paste data into a workspace. */
+export interface IPaster<U extends CopyData, T extends ICopyable> {
+  paste(
+    copyData: U,
+    workspace: WorkspaceSvg,
+    coordinate?: Coordinate,
+  ): T | null;
+}
+
+/** @returns True if the given object is a paster. */
+export function isPaster(obj: any): obj is IPaster<CopyData, ICopyable> {
+  return obj.paste !== undefined;
+}