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/Folding.tsx

@@ -0,0 +1,75 @@
+import { Plugin } from "prosemirror-state";
+import { findBlockNodes } from "prosemirror-utils";
+import { Decoration, DecorationSet } from "prosemirror-view";
+import Storage from "../../utils/Storage";
+import Extension from "../lib/Extension";
+import { headingToPersistenceKey } from "../lib/headingToSlug";
+import findCollapsedNodes from "../queries/findCollapsedNodes";
+
+export default class Folding extends Extension {
+  get name() {
+    return "folding";
+  }
+
+  get plugins() {
+    let loaded = false;
+
+    return [
+      new Plugin({
+        view: (view) => {
+          loaded = false;
+          view.dispatch(view.state.tr.setMeta("folding", { loaded: true }));
+          return {};
+        },
+        appendTransaction: (transactions, oldState, newState) => {
+          if (loaded) {
+            return;
+          }
+          if (
+            !transactions.some((transaction) => transaction.getMeta("folding"))
+          ) {
+            return;
+          }
+
+          let modified = false;
+          const tr = newState.tr;
+          const blocks = findBlockNodes(newState.doc);
+
+          for (const block of blocks) {
+            if (block.node.type.name === "heading") {
+              const persistKey = headingToPersistenceKey(
+                block.node,
+                this.editor.props.id
+              );
+              const persistedState = Storage.get(persistKey);
+
+              if (persistedState === "collapsed") {
+                tr.setNodeMarkup(block.pos, undefined, {
+                  ...block.node.attrs,
+                  collapsed: true,
+                });
+                modified = true;
+              }
+            }
+          }
+
+          loaded = true;
+          return modified ? tr : null;
+        },
+        props: {
+          decorations: (state) => {
+            const { doc } = state;
+            const decorations: Decoration[] = findCollapsedNodes(doc).map(
+              (block) =>
+                Decoration.node(block.pos, block.pos + block.node.nodeSize, {
+                  class: "folded-content",
+                })
+            );
+
+            return DecorationSet.create(doc, decorations);
+          },
+        },
+      }),
+    ];
+  }
+}