Migrate to top-level `browserslistEnv` option

[Billy Janitsch]

See the docs and the announcement.

Going forward, this will be the preferred way to override the browserslist env, since it integrates with plugins/presets beyond @babel/preset-env such as the new polyfill transforms.

Unfortunately, it's not a straightforward migration because the top-level option can't be specified in presets. I opened babel/babel#13317 to see whether this might change. If not, we'll need to add at least some config to each app.