Kernel: Scheduler can now check if tid is valid

Tid can become invalid if the thread is already terminated

kernel/include/kernel/Scheduler.h

@@ -31,6 +31,8 @@ namespace Kernel
 		Thread& current_thread();
 		static pid_t current_tid();
 
+		static bool is_valid_tid(pid_t tid);
+
 	private:
 		Scheduler() = default;