Set default CameraFit maxZoom values to infinity

The other parameters for CameraFit all have default values which will
not cause changes to the calculated CameraFit (i.e. padding is zero,
forceIntegerZoomLevel is false). Setting maxZoom to infinity makes it
consistent with the other parameters in that it will not affect the
calculated CameraFit.

lib/src/map/camera/camera_fit.dart

@@ -82,7 +82,7 @@ class FitBounds extends CameraFit {
   const FitBounds._({
     required this.bounds,
     this.padding = EdgeInsets.zero,
-    this.maxZoom = 18,
+    this.maxZoom = double.infinity,
     this.forceIntegerZoomLevel = false,
   });
 
@@ -181,7 +181,7 @@ class FitInsideBounds extends CameraFit {
   const FitInsideBounds._({
     required this.bounds,
     this.padding = EdgeInsets.zero,
-    this.maxZoom = 18,
+    this.maxZoom = double.infinity,
     this.forceIntegerZoomLevel = false,
   });
 
@@ -373,7 +373,7 @@ class FitCoordinates extends CameraFit {
   const FitCoordinates._({
     required this.coordinates,
     this.padding = EdgeInsets.zero,
-    this.maxZoom = 18,
+    this.maxZoom = double.infinity,
     this.forceIntegerZoomLevel = false,
   });