basic reading of battery level (#182)

libs/core/battery.ts

@@ -0,0 +1,12 @@
+
+namespace brick {
+    /**
+     * Returns the current battery level
+     */
+    //% blockId=brickBatteryLevel block="battery level"
+    //% group="More"
+    export function batteryLevel(): number {
+        const info = sensors.internal.getBatteryInfo();
+        return info.current;
+    }
+}