Move the scheduler code to its own class.

This is very mechanical.

Kernel/Scheduler.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <AK/Assertions.h>
+class Process;
+
+extern Process* current;
+
+class Scheduler {
+public:
+    static void initialize();
+    static bool pick_next();
+    static void pick_next_and_switch_now();
+    static void switch_now();
+    static bool yield();
+    static bool context_switch(Process&);
+    static void prepare_for_iret_to_new_process();
+    static void prepare_to_modify_own_tss();
+};
+
+int sched_yield();