Added a module to configure browser based on the gecko engine, such as Firefox.

This module should provide similar functionality to the firefox module in
home-manager. Some notable differences between the two include:

* home-manager configures a single browser. This means that any configuration
  that cannot be done on a per-profile basis is shared between all
  profiles. This module configures a new copy of the browser for every profile,
  ensuring that *all* configuration can be on a per-profile basis.

  This might be seen as insanity in a regular distro, but in NixOS this is
  trivial to do and requires no extra storage space.

* home-manager modifies files in the user's directory to configure things such
  as extensions and search engines. This module avoids that when possible by
  pushing configuration into policies and preferences at a browser level.
  This is much nicer for impermanence-based systems.

modules/home-manager/gecko-browser/gecko-lib/prefs.nix

@@ -0,0 +1,49 @@
+{
+  pkgs,
+  lib,
+  info,
+  ...
+}:
+
+let
+  type = lib.types.attrsOf (
+    lib.types.oneOf [
+      lib.types.int
+      lib.types.bool
+      lib.types.str
+    ]
+  );
+in
+{
+  inherit type;
+
+  options = {
+    prefs = lib.mkOption {
+      inherit type;
+      default = { };
+      description = "User preferences.";
+    };
+
+    lockPref = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "When adding user preferences, determine whether to use lockPref over user_pref";
+    };
+  };
+
+  postprocess =
+    { lockPref, prefs, ... }:
+    {
+      prefs =
+        builtins.concatStringsSep "\n"
+        <| (x: [ "// Generated by ${info.name} module" ] ++ x)
+        <| lib.mapAttrsToList (
+          let
+            prefFunc = if lockPref then "lockPref" else "user_pref";
+          in
+          key: val: ''${prefFunc}("${key}", ${builtins.toJSON val});''
+        )
+        <| prefs;
+    };
+}