Move WASI types into separate header (#2724)

To allow anything to depend on WASI types, including platform-specific
data structures, move the WASI libc filesystem/clock interface into
`platform_api_extension.h`, which leaves just WASI types in
`platform_wasi.h`. And `platform_wasi.h` has been renamed to
`platform_wasi_types.h` to reflect that it only defines types now and no
function declarations. Finally, these changes allow us to remove the
`windows_fdflags` type which was essentially a duplicate of
`__wasi_fdflags_t`.

core/shared/platform/common/libc-util/libc_errno.h

@@ -6,7 +6,7 @@
 #ifndef WASI_ERRNO_H
 #define WASI_ERRNO_H
 
-#include "platform_wasi.h"
+#include "platform_wasi_types.h"
 
 // Converts an errno error code to a WASI error code.
 __wasi_errno_t