feat: Warn user if they're about to close a browser tab with unsaved Penrose code

[Keenan Crane]

Would we want some kind of warning if a user is about to close a Penrose tab they've typed a bunch of code into?

E.g., https://stackoverflow.com/questions/2858057/ask-for-confirm-when-closing-a-tab

[Wode "Nimo" Ni]

Mega-issue on this: #1683

there's no confirmation that you're abandoning work if you navigate to someone else's gist while in draft mode
there's no confirmation that you're abandoning work if you navigate to new webpage while in draft mode

The PR #1696 attempted this issue, but beforeunload seemed to be a blocker. Need to investigate again.