Migrate to floating UI

[benmccann]

Floating UI is the newer version of popper. Using it will avoid all of the issues around CJS/ESM as it's properly packaged using exports

I took a bit of a stab at it, but ran out of time. I thought I'd share here as a starting point for anyone who wants to take a stab at migrating off popper.js

benmccann@46d9f94

The initial styles aren't being applied correctly in my branch, which means that the popper ends up at the bottom of the screen. I think that's the main thing to fix. I also didn't try to migrate the dropdown - though that could possibly be done in a separate PR if we don't mind having two dependencies in the interim.