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/hooks/useTextSelection.ts

@@ -0,0 +1,22 @@
+import * as React from "react";
+import useEventListener from "./useEventListener";
+
+/**
+ * A hook that returns the currently selected text.
+ *
+ * @returns The selected text
+ */
+export default function useTextSelection() {
+  const [selection, setSelection] = React.useState<string>("");
+
+  const handleMouse = React.useCallback(() => {
+    const selection = window.getSelection();
+    const text = selection?.toString();
+    setSelection(text ?? "");
+  }, []);
+
+  useEventListener("mousemove", handleMouse);
+  useEventListener("mouseup", handleMouse);
+
+  return selection;
+}