Add IRQHandler class that can be subclasses to handle an IRQ.

Also move Keyboard to a class implementation using this pattern.

Kernel/IRQHandler.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <AK/Types.h>
+
+class IRQHandler {
+public:
+    virtual ~IRQHandler();
+    virtual void handleIRQ() = 0;
+
+    byte irqNumber() const { return m_irqNumber; }
+
+    void enableIRQ();
+    void disableIRQ();
+
+protected:
+    explicit IRQHandler(byte irq);
+
+private:
+    byte m_irqNumber { 0 };
+};
+