Allow querying int and string prefs too via functional syntax:
```
@media -moz-pref("pref-name", <value>) {
```
No argument means the pref exists and has a non-zero/false/empty value.
Unfortunately stylelint is still unhappy with this so we need to keep
the annoying comments...
Differential Revision: https://phabricator.services.mozilla.com/D237275
c67d8c9c9c