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/endpoints.dtsi

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