[cmd3] Enforce command lifetimes across all opmode and command scopes (#8705)

Commands are no longer able to outlive their schedule-site's scope,
regardless of how they were scheduled (set as a default command, bound
to a trigger, or manually scheduled)

As a consequence, default commands need better tracking so the default
command setting can be released when their scope exits and the next-most
appropriate default command can be rescheduled (eg, an opmode sets a
default command, then the globally-scoped default is restored when the
opmode exits). Some complexity is required here to make it work well for
edge cases.

Like `schedule()`, `setDefaultCommand()` will immediately start the new
default command if called inside of another command to avoid 1-loop
delays. However, this does not apply when called by the _current_
default command, as it would result in attempting to cancel the default
command while it's mounted (which is impossible and would throw an
exception)

```java
class Robot extends OpModeRobot {
  final Drive drive = new Drive();
  final CommandXboxController controller = new CommandXboxController(1);

  public Robot() {
    // global default command, active unless overridden in an opmode or command
    drive.setDefaultCommand(drive.stop());

    // global trigger binding, always active
    controller.rightBumper().onTrue(drive.setX());
  }
}

@Teleop
class ExampleOpMode extends PeriodicOpMode {
  public ExampleOpMode(Robot robot) {
    // opmode-specific default command
    robot.drive.setDefaultCommand(robot.drive.operatorControl(robot.controller));

    // opmode-specific binding
    robot.controller.leftBumper().whileTrue(robot.drive.stop());

    // opmode-specific binding that takes precedence over the global binding
    // because it happens last; it "wins out" over the `setX()` binding
    robot.controller.rightBumper().onTrue(robot.drive.selfTest());
  }

  @Override
  public void periodic() {
    Scheduler.getDefault().run();
  }
}
```

wpilibc/src/main/native/include/wpi/driverstation/DriverStation.hpp

@@ -454,6 +454,20 @@ class DriverStation final {
    */
   static void ClearOpModes();
 
+  /**
+   * Sets the program starting flag in the DS. This will also allow
+   * getOpModeId() and getOpMode() to return values for the selected
+   * OpMode in the DS application, if the DS is connected by the time this
+   * method is called.
+   *
+   * <p>Most users will not need to use this method; the TimedRobot and
+   * OpModeRobot robot framework classes will call it automatically after
+   * the main robot class is instantiated.
+   *
+   * <p>This is what changes the DS to showing robot code ready.
+   */
+  static void ObserveUserProgramStarting();
+
   /**
    * Gets the operating mode selected on the driver station. Note this does not
    * mean the robot is enabled; use IsEnabled() for that. In a match, this will
@@ -464,7 +478,7 @@ class DriverStation final {
    * @return the unique ID provided by the AddOpMode() function; may return 0 or
    * a unique ID not added, so callers should be prepared to handle that case
    */
-  static int64_t GetOpModeId() { return GetControlWord().GetOpModeId(); }
+  static int64_t GetOpModeId();
 
   /**
    * Gets the operating mode selected on the driver station. Note this does not