Draft: Rewrite pacrunner to not use libproxy

Red Hat wants to get rid of libproxy, but we do not want to get rid of proxy autoconfig. So let's do it ourselves. It's not hard.

• Fixes #163 (closed)
• Fixes #28 (closed)
• Fixes #5

TODO:

• Update CI runners to pass CI
• Wants a couple simple tests
• Wants a bubblewrap sandbox
• Cache the pac file instead of downloading it again every time
• Wants newer Javascript utils (current upstream)
• Modify gnome-control-center UI to require PAC URL and remove reference to web proxy autodiscovery (no longer supported)