feat: Add import/export of documents as JSON (#4621)

* feat: Add export of documents as JSON

* Rename, add structured collection description

* stash

* ui

* Add entity creation data to JSON archive

* Import JSON UI plumbing

* stash

* Messy, but working

* tsc

* tsc

shared/editor/lib/markdown/serializer.ts

@@ -50,7 +50,7 @@ export class MarkdownSerializer {
   // :: (Node, ?Object) → string
   // Serialize the content of the given node to
   // [CommonMark](http://commonmark.org/).
-  serialize(content, options?: { tightLists?: boolean }) {
+  serialize(content, options?: { tightLists?: boolean }): string {
     const state = new MarkdownSerializerState(this.nodes, this.marks, options);
     state.renderContent(content);
     return state.out;