chromium: Allow to put extensions in system path

This should allow us to easily add system-wide Chromium extensions via a
NixOS configuration similar to this:

{ pkgs, ... }: {
  environment.pathsToLink = [ "/share/chromium/extensions" ];
  environment.systemPackages = [ pkgs.my-shiny-extension ];
}

For more details about what Chromium expects within that directory, see:

https://developer.chrome.com/extensions/external_extensions



I've introduced this because of a personal desire to gain more control
about which extensions are installed and what they are able to do. All
of the extensions I use are free software, but despite that it's useful
to either easily patch them and also prevent unwanted automatic updates.

Tested this using the NixOS "chromium.stable" test on x86_64-linux.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @offlinehacker because of #21050