Add document insights panel (#4418)

* Add document context to allow accessing editor in header, modals, and elsewhere

* lint

* framework

* Hacking together fast

* Insights

* Spacing tweak, docs

app/components/DocumentContext.ts

@@ -0,0 +1,19 @@
+import * as React from "react";
+import { Editor } from "~/editor";
+
+export type DocumentContextValue = {
+  /** The current editor instance for this document. */
+  editor: Editor | null;
+  /** Set the current editor instance for this document. */
+  setEditor: (editor: Editor) => void;
+};
+
+const DocumentContext = React.createContext<DocumentContextValue>({
+  editor: null,
+  // eslint-disable-next-line @typescript-eslint/no-empty-function
+  setEditor() {},
+});
+
+export const useDocumentContext = () => React.useContext(DocumentContext);
+
+export default DocumentContext;