Fix remove_directory_trailing_slashes on Windows

This commit provides a fix for `remove_directory_trailing_slashes`
test case on Windows. It adds a missing mapping between the following
WinAPI error code and WASI error:

```
ERROR_DIRECTORY => __WASI_ENOTDIR
```

where `ERROR_DIRECTORY` is thrown when the directory name is invalid.

winx/src/winerror.rs

@@ -84,6 +84,8 @@ win_error_expand! {
     ERROR_NOT_A_REPARSE_POINT,
     /// An attempt was made to move the file pointer before the beginning of the file.
     ERROR_NEGATIVE_SEEK,
+    /// The directory name is invalid.
+    ERROR_DIRECTORY,
 }
 
 impl WinError {