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/search.nix

@@ -0,0 +1,118 @@
+{ lib, ... }:
+
+let
+  submodule =
+    { config, ... }:
+    {
+      options = {
+        default = lib.mkOption {
+          type = lib.types.nullOr lib.types.str;
+          default = null;
+          example = "ddg";
+          description = "The default search engine used in the address bar and search bar.";
+        };
+
+        engines = lib.mkOption {
+          type = lib.types.attrsOf (lib.types.submodule engineSubmodule);
+          default = { };
+        };
+      };
+    };
+
+  engineSubmodule =
+    { name, ... }:
+    {
+      options = {
+        name = lib.mkOption {
+          type = lib.types.str;
+          default = name;
+        };
+
+        searchUri = {
+          base = lib.mkOption {
+            type = lib.types.strMatching "[a-z]+://[-A-Za-z0-9.]+(:[0-9]+)?(/[^?]*)?";
+          };
+
+          params = lib.mkOption {
+            type = lib.types.listOf (
+              lib.types.submodule {
+                options = {
+                  key = lib.mkOption { type = lib.types.str; };
+                  value = lib.mkOption { type = lib.types.str; };
+                };
+              }
+            );
+            default = [ ];
+          };
+        };
+
+        method = lib.mkOption {
+          type = lib.types.enum [
+            "GET"
+            "POST"
+          ];
+          default = "GET";
+        };
+
+        alias = lib.mkOption {
+          type = lib.types.nullOr lib.types.str;
+          default = null;
+        };
+
+        icon = lib.mkOption {
+          type = lib.types.nullOr (
+            lib.types.oneOf [
+              lib.types.package
+              lib.types.str
+            ]
+          );
+          default = null;
+        };
+      };
+    };
+
+  buildPolicyFromEngine =
+    {
+      name,
+      searchUri,
+      method,
+      alias,
+      icon,
+    }:
+    let
+      params =
+        builtins.concatStringsSep "&" <| builtins.map (x: "${x.key}=${x.value}") <| searchUri.params;
+    in
+    {
+      Name = name;
+      URLTemplate =
+        if method == "POST" then
+          searchUri.base
+        else if params == "" then
+          searchUri.base
+        else
+          "${searchUri.base}?${params}";
+      Method = method;
+      ${if method == "POST" then "PostData" else null} = params;
+      ${if icon != null then "IconURL" else null} = "file://${builtins.toString icon}";
+      ${if alias != null then "Alias" else null} = alias;
+    };
+in
+{
+  options = {
+    search = lib.mkOption {
+      type = lib.types.submodule submodule;
+      default = { };
+      description = "Declarative search engine configuration.";
+    };
+  };
+
+  process =
+    { search, ... }:
+    {
+      policies.SearchEngines = {
+        ${if search.default == null then null else "Default"} = search.default;
+        Add = lib.mapAttrsToList (_: buildPolicyFromEngine) search.engines;
+      };
+    };
+}