feat: Make limit field mutable in the Timer class (#2358)

As stated in #2357, the field `limit` of the class `Timer` had no reason
to be final. By removing this keyword, we make it possible to add more
time to a given `Timer` object (so that they can be used as a countdown
for example).

packages/flame/lib/src/timer.dart

@@ -5,8 +5,12 @@ import 'dart:ui';
 /// interval like events.
 ///
 /// Timer auto-starts by default.
+///
+/// NOTE: You can change the [limit], but keep in mind that the timer
+/// won't start automatically if the limit is raised and the timer currently
+/// is stopped.
 class Timer {
- final double limit;
+ double limit;
  VoidCallback? onTick;
  bool repeat;
  double _current = 0;
@@ -19,7 +23,7 @@ class Timer {
  bool autoStart = true,
  }) : _running = autoStart;
 
- /// The current amount of ms that has passed on this iteration
+ /// The current amount of seconds that has passed on this iteration
  double get current => _current;
 
  /// If the timer is finished, timers that repeat never finish