nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/hardware/bluetooth/default.nix

@@ -0,0 +1,56 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.hardware.bluetooth;
+in
+{
+  options.my.hardware.bluetooth = with lib; {
+    enable = mkEnableOption "bluetooth configuration";
+
+    enableHeadsetIntegration = my.mkDisableOption "A2DP sink configuration";
+
+    loadExtraCodecs = my.mkDisableOption "extra audio codecs";
+  };
+
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    # Enable bluetooth devices and GUI to connect to them
+    {
+      hardware.bluetooth.enable = true;
+      services.blueman.enable = true;
+    }
+
+    # Support for additional bluetooth codecs
+    (lib.mkIf cfg.loadExtraCodecs {
+      hardware.pulseaudio = {
+        extraModules = [ pkgs.pulseaudio-modules-bt ];
+        package = pkgs.pulseaudioFull;
+      };
+
+      environment.etc = {
+        "wireplumber/bluetooth.lua.d/51-bluez-config.lua".text = ''
+          bluez_monitor.properties = {
+            -- SBC XQ provides better audio
+            ["bluez5.enable-sbc-xq"] = true,
+
+            -- mSBC provides better audio + microphone
+            ["bluez5.enable-msbc"] = true,
+
+            -- Synchronize volume with bluetooth device
+            ["bluez5.enable-hw-volume"] = true,
+
+            -- FIXME: Some devices may now support both hsp_ag and hfp_ag
+            ["bluez5.headset-roles"] = "[ hsp_hs hsp_ag hfp_hf hfp_ag ]"
+          }
+        '';
+      };
+    })
+
+    # Support for A2DP audio profile
+    (lib.mkIf cfg.enableHeadsetIntegration {
+      hardware.bluetooth.settings = {
+        General = {
+          Enable = "Source,Sink,Media,Socket";
+        };
+      };
+    })
+  ]);
+}