Fixed theme

- fixed: currentMode, dark, light now actually emit new values when changed
- added: getters for lightTheme, darkTheme, and systemTheme
- breaking: currentMode -> systemTheme
- added: currentTheme
- breaking: currentMode => currentTheme
- change: Arcane.theme.followSystemTheme(context) required to follow system theme
- change: After following the system theme, calling Arcane.theme.switchTheme() cancels following the system theme
- added: ArcaneReactiveTheme is now registered in ArcaneServiceProvider when using ArcaneApp

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

lib/src/arcane_app.dart

@@ -46,7 +46,10 @@ class ArcaneApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return ArcaneEnvironmentProvider(
       child: ArcaneServiceProvider(
-        serviceInstances: services,
+        serviceInstances: [
+          ArcaneReactiveTheme.I,
+          ...services,
+        ],
         child: child,
       ),
     );