Support whitespace stripping for JSON resources

Similarly to how glib-compile-resources can call xmllint to eliminate
whitespace in XML files to reduce their size inside a GResource, we can
use json-glib-format to achieve the same result.

The mechanism for using json-glib-format is the same, with a separate
environment variable if we want to direct glib-compile-resources to a
version of json-glib-format that is not the one in the PATH.

https://bugzilla.gnome.org/show_bug.cgi?id=794284