[commands] Add Subsystem.idle() (#7815)

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/Subsystem.java

@@ -98,6 +98,15 @@ public interface Subsystem {
     CommandScheduler.getInstance().registerSubsystem(this);
   }
 
+  /**
+   * Constructs a command that does nothing until interrupted. Requires this subsystem.
+   *
+   * @return the command
+   */
+  default Command idle() {
+    return Commands.idle(this);
+  }
+
   /**
    * Constructs a command that runs an action once and finishes. Requires this subsystem.
    *