(core) Add April Fools easter egg

Summary: What happens when you type "rr" instead of "r" in an anchor link's row number?

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3829

app/common/Prefs.ts

@@ -85,6 +85,7 @@ export const BehavioralPrompt = StringUnion(
   'pageWidgetPickerSelectBy',
   'editCardLayout',
   'addNew',
+  'rickRow',
 );
 export type BehavioralPrompt = typeof BehavioralPrompt.type;