Shell: Implement a "cd history" (cdh) builtin

`cdh` with no arguments dumps the last 8 cd calls in history, and
`cdh [index]` can be used to cd to re-run a specific index from that
history. `cdh` itself it a thin wrapper of the `cd` builtin.

There's definitely some improvements that can be made for this command,
but this seems like a good starting point for getting a feel for it and
ideas for changing it in the future.

It's not entirely clear whether we should be storing the resolved path -
or simply just the last argument passed to cd. For now we just use the
last path passed into cd as this seemed like the better option for now.

This means:
 * invalid paths will still be stored in history (potentially useful)
 * cdh's can be repeated for duplicate directory names
 * the history looks a little nicer on the eyes

It might make sense to use resolved paths.

Closes #397

Shell/GlobalState.h

@@ -28,6 +28,7 @@
 
 #include <AK/String.h>
 #include <AK/Vector.h>
+#include <AK/CircularQueue.h>
 #include <termios.h>
 
 struct GlobalState {
@@ -43,6 +44,7 @@ struct GlobalState {
     bool was_resized { false };
     int last_return_code { 0 };
     Vector<String> directory_stack;
+    CircularQueue<String, 8> cd_history; // FIXME: have a configurable cd history length
 };
 
 extern GlobalState g;