Abstract POSIX filesystem functions (#2585)

To allow non-POSIX platforms such as Windows to support WASI libc
filesystem functionality, create a set of wrapper functions which provide a
platform-agnostic interface to interact with the host filesystem. For now,
the Windows implementation is stubbed but this will be implemented
properly in a future PR. There are no functional changes in this change,
just a reorganization of code to move any direct POSIX references out of
posix.c in the libc implementation into posix_file.c under the shared
POSIX sources.

See https://github.com/bytecodealliance/wasm-micro-runtime/issues/2495 for a
more detailed overview of the plan to port the WASI libc filesystem to Windows.

core/shared/platform/include/platform_api_extension.h

@@ -7,6 +7,7 @@
 #define PLATFORM_API_EXTENSION_H
 
 #include "platform_common.h"
+#include "platform_wasi.h"
 /**
  * The related data structures should be defined
  * in platform_internal.h