Adds support for Firefox forks by introducing methods that create
generic configs and options. Additional configs and options can be
added in separate modules.
This commit makes it possible to specify Firefox' extraPolicies
through:
programs.firefox.package = pkgs.firefox.override {
extraPolicies = {... }
}
This was possible in the past but was broken by:
3feeb77155
firefox: add support for specifying policies (#4626)
