chore: Editor 'plugin' -> 'extension'

They've always been called extensions, not sure why the folder was plugins. Part of ongoing spring cleaning

shared/editor/extensions/ClipboardTextSerializer.ts

@@ -0,0 +1,38 @@
+import { Plugin, PluginKey } from "prosemirror-state";
+import Extension from "../lib/Extension";
+import textBetween from "../lib/textBetween";
+
+/**
+ * A plugin that allows overriding the default behavior of the editor to allow
+ * copying text for nodes that do not inherently have text children by defining
+ * a `toPlainText` method in the node spec.
+ */
+export default class ClipboardTextSerializer extends Extension {
+  get name() {
+    return "clipboardTextSerializer";
+  }
+
+  get plugins() {
+    const textSerializers = Object.fromEntries(
+      Object.entries(this.editor.schema.nodes)
+        .filter(([, node]) => node.spec.toPlainText)
+        .map(([name, node]) => [name, node.spec.toPlainText])
+    );
+
+    return [
+      new Plugin({
+        key: new PluginKey("clipboardTextSerializer"),
+        props: {
+          clipboardTextSerializer: () => {
+            const { doc, selection } = this.editor.view.state;
+            const { ranges } = selection;
+            const from = Math.min(...ranges.map((range) => range.$from.pos));
+            const to = Math.max(...ranges.map((range) => range.$to.pos));
+
+            return textBetween(doc, from, to, textSerializers);
+          },
+        },
+      }),
+    ];
+  }
+}