AK+LibIPC: Make all enums codable

If an enum has a supported underlying type we can provide encoding and
decoding for the enum as well.

Userland/Libraries/LibIPC/Encoder.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Concepts.h>
+#include <AK/StdLibExtras.h>
 #include <LibIPC/Forward.h>
 #include <LibIPC/Message.h>
 
@@ -62,6 +64,13 @@ public:
         return *this;
     }
 
+    template<Enum T>
+    Encoder& operator<<(T const& enum_value)
+    {
+        *this << AK::to_underlying(enum_value);
+        return *this;
+    }
+
     template<typename T>
     Encoder& operator<<(const T& value)
     {