Remove api from include path, add redirects

Can't have cores/rp2040/api in the include path because the Arduino API
dir has a "String.h" file.  On Windows, because it is case-insensitive
normally, this overrides the POSIX "string.h" header leading to bad
stuff.

Add manual redirect includes for commonly accessed headers in the
cores/rp2040 path instead.

cores/rp2040/Client.h

+#include "api/Client.h"

cores/rp2040/IPAddress.h

+#include "api/IPAddress.h"

cores/rp2040/Interrupts.h

+#include "api/Interrupts.h"

cores/rp2040/Print.h

+#include "api/Print.h"

cores/rp2040/Server.h

+#include "api/Server.h"

cores/rp2040/Stream.h

+#include "api/String.h"

cores/rp2040/Udp.h

+#include "api/Udp.h"