Add a prompt for unsaved changes to `beforeunload`

Summary I tried looking through the issues and discussions for this. Closest case I could find is it proposes another solution in a somewhat different context. --- I had a content editor contact me about some changes they apparently lost. In their scenario, they made some changes or added a linked entity (the one for which a side panel opens), but it was not entirely obvious to them that such action may necessitate saving the root entity as well. In our case, forms are not too big, but related entity editing can make current application state unclear to users (after all it seems like you clicked a checkmark button called "save"). When you try to navigate away from the form within the app, directus UI will helpfully ask you to stay or discard the changes. If you accidentally navigate away from directus within the tab, you can go back and recover form state, which is also useful and seems to work for the entire tab history (although it can behave not entirely obviou…