feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

app/dts/behaviors.dtsi

@@ -10,3 +10,4 @@
 #include <behaviors/rgb_underglow.dtsi>
 #include <behaviors/bluetooth.dtsi>
 #include <behaviors/ext_power.dtsi>
+#include <behaviors/endpoints.dtsi>

app/dts/behaviors/endpoints.dtsi

@@ -0,0 +1,9 @@
+/ {
+    behaviors {
+        end: behavior_endpoints {
+            compatible = "zmk,behavior-endpoints";
+            label = "ENDPOINTS";
+            #binding-cells = <1>;
+        };
+    };
+};

app/dts/bindings/behaviors/zmk,behavior-endpoints.yaml

@@ -0,0 +1,10 @@
+#
+# Copyright (c) 2020, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+#
+
+description: Endpoints Behavior
+
+compatible: "zmk,behavior-endpoints"
+
+include: one_param.yaml