* Improved Scheduler API
- Added `Scheduler#builder(plugin)`
This method allows a more simplified builder while maintaining the main requirement of the executor plugin
- Added `Scheduler#taskByPlugin(plugin)`
Allows to obtain the tasks that a plugin has sent to execute and that are currently active
- Added `TaskBuilder#task(Consumer<SchuledTask>)`
Allows to specify a task with access to the task itself with the ability to cancel itself
* Applied requested changes
- Removed tasks builder method
- Added `Scheduler#buildTask(plugin, Consumer<ScheduledTask>)`
* Removed some unused imports
* Applied suggested change
* Fix possible test bug
* Applied more suggested changes
* Fixed tests inside tasks
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