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/Placeholder.ts

@@ -0,0 +1,50 @@
+import { Plugin } from "prosemirror-state";
+import { Decoration, DecorationSet } from "prosemirror-view";
+import Extension from "../lib/Extension";
+
+export default class Placeholder extends Extension {
+  get name() {
+    return "empty-placeholder";
+  }
+
+  get defaultOptions() {
+    return {
+      emptyNodeClass: "placeholder",
+      placeholder: "",
+    };
+  }
+
+  get plugins() {
+    return [
+      new Plugin({
+        props: {
+          decorations: (state) => {
+            const { doc } = state;
+            const decorations: Decoration[] = [];
+            const completelyEmpty =
+              doc.textContent === "" &&
+              doc.childCount <= 1 &&
+              doc.content.size <= 2;
+
+            doc.descendants((node, pos) => {
+              if (!completelyEmpty) {
+                return;
+              }
+              if (pos !== 0 || node.type.name !== "paragraph") {
+                return;
+              }
+
+              const decoration = Decoration.node(pos, pos + node.nodeSize, {
+                class: this.options.emptyNodeClass,
+                "data-empty-text": this.options.placeholder,
+              });
+              decorations.push(decoration);
+            });
+
+            return DecorationSet.create(doc, decorations);
+          },
+        },
+      }),
+    ];
+  }
+}