LibCore: Introduce a C_OBJECT macro.

This macro goes at the top of every CObject-derived class like so:

class SomeClass : public CObject {
    C_OBJECT(SomeClass)
public:
    ...

At the moment, all it does is create an override for the class_name() getter
but in the future this will be used to automatically insert member functions
into these classes.

Libraries/LibCore/CTCPSocket.h

@@ -1,6 +1,7 @@
 #include <LibCore/CSocket.h>
 
 class CTCPSocket final : public CSocket {
+    C_OBJECT(CTCPSocket)
 public:
     explicit CTCPSocket(CObject* parent = nullptr);
     virtual ~CTCPSocket() override;