png: Split library to thirdparty dir and allow unbundling

Uses the new structure agreed upon in #6157, but the thirdparty/ folder
does not behave following a logic similar to that of modules/ yet.

The png driver can't be moved to a module as discussed in #6157, as it's
required by core together with a few other ImageLoader implementations
(see drivers/register_driver_types.cpp:register_core_driver_types())

Dropped the possibility to disable PNG support, it's a core component
of Godot.

drivers/png/image_loader_png.cpp

@@ -30,6 +30,7 @@
 
 #include "print_string.h"
 #include "os/os.h"
+
 #include <string.h>