Bug 1666734 - Add two-sided printing checkbox to print UI r=sfoster,fluent-reviewers,flod,AlaskanEmily

This patch adds a new checkbox to the print UI for two-sided printing.
The checkbox is only visible if the currently selected printer supports
two-sided printing.

Notable Changes:
  - Add new section and checkbox for two-sided printing.
  - Add new getter to settings proxy for supportsDuplex.
  - Add new setter/getter to settings proxy for printDuplex.
  - Add new test for no duplex with PDF printer.
  - Add new test for toggle duplex in portrait orientation.
  - Add new test for toggle duplex in landscape orientation.
  - Add new test for toggle orientation with duplex checked.
  - Correctly set duple mode in GTK print settings.

Depends on D94026

Differential Revision: https://phabricator.services.mozilla.com/D93621