Drop support for Internet Explorer (#1313)

WordPress abandoned the support of IE with WP5.8 in July 2021.
https://wordpress.org/news/2021/05/dropping-support-for-internet-explorer-11/

Though we don't require that version of WP, it's time to abandon
that legacy. See also this very old ticket: #32.

1) Remove workaround for `RegExp:@@split`
See also https://caniuse.com/mdn-javascript_builtins_regexp_--split

2) Update our `browserslist` for JS to be synced with WordPress
https://github.com/WordPress/wordpress-develop/blob/trunk/package.json

3) Add more debug info about browser in the troubleshooting section.