AK: Resolve format related circular dependencies properly.

With this commit, <AK/Format.h> has a more supportive role and isn't
used directly.

Essentially, there now is a public 'vformat' function ('v' for vector)
which takes already type erased parameters. The name is choosen to
indicate that this function behaves similar to C-style functions taking
a va_list equivalent.

The interface for frontend users are now 'String::formatted' and
'StringBuilder::appendff'.

AK/LogStream.cpp

@@ -27,6 +27,7 @@
 #include <AK/FlyString.h>
 #include <AK/LogStream.h>
 #include <AK/String.h>
+#include <AK/StringBuilder.h>
 #include <AK/StringView.h>
 
 #ifdef KERNEL