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/windows/platform_internal.h

@@ -32,6 +32,8 @@
 #include <basetsd.h>
 #include <signal.h>
 
+#include "platform_wasi_types.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -146,19 +148,9 @@ typedef enum windows_access_mode {
     windows_access_mode_write = 1 << 1
 } windows_access_mode;
 
-// These enum values are defined to be the same as the corresponding WASI
-// fdflags so they can be used interchangeably.
-typedef enum windows_fdflags {
-    windows_fdflags_append = 1 << 0,
-    windows_fdflags_dsync = 1 << 1,
-    windows_fdflags_nonblock = 1 << 2,
-    windows_fdflags_rsync = 1 << 3,
-    windows_fdflags_sync = 1 << 4
-} windows_fdflags;
-
 typedef struct windows_handle {
     windows_handle_type type;
-    windows_fdflags fdflags;
+    __wasi_fdflags_t fdflags;
     windows_access_mode access_mode;
     union {
         HANDLE handle;

core/shared/platform/windows/win_file.c

@@ -1046,7 +1046,7 @@ os_writev(os_file_handle handle, const struct __wasi_ciovec_t *iov, int iovcnt,
 {
     CHECK_VALID_HANDLE(handle);
 
-    bool append = (handle->fdflags & windows_fdflags_append) != 0;
+    bool append = (handle->fdflags & __WASI_FDFLAG_APPEND) != 0;
     LARGE_INTEGER write_offset = { .QuadPart = 0 };
     DWORD move_method = append ? FILE_END : FILE_CURRENT;
 

core/shared/platform/windows/win_util.h

@@ -6,7 +6,7 @@
 #ifndef _WIN_UTIL_H
 #define _WIN_UTIL_H
 
-#include "platform_wasi.h"
+#include "platform_wasi_types.h"
 #include "windows.h"
 
 __wasi_timestamp_t