Add RegExp dotAll feature test
Add RegExp dotAll feature test

As mentioned in
https://github.com/vector-im/element-web/issues/16020#issuecomment-774384598,
we're depending on the `dotAll` / `s` regex feature. This adds a test so
browsers without this see an error screen.

vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web