fix(plugins): improve user agent selector

[blakebyrnes]

The user agent selector has a few holes that are plugged by this PR.

1. The default example in the docs ~mac and chrome >90 did not work
2. If you had multiple selectors (ie, OS and browser, it would take matches of "either/or", which was not what we wanted)
3. There are issues if you use an OS we don't know about.
4. We needed default selectors if you don't provide an operator (=) or version (*)

Closes #369 and causes #371 to emit a better error, but we probably need to follow up with some kind of actual option to use a mobile user agent on a default browser.