Fixes path_symlink_trailing_slashes on Windows

This commit:
* adds missing `ERROR_ALREADY_EXISTS => __WASI_EEXIST` mapping
* re-routes Win errors into correct WASI values in `symlink_*`
  fns when target exists and/or contains a trailing slash
* remaps `ERROR_INVALID_NAME => __WASI_ENOENT`

winx/src/winerror.rs

@@ -86,6 +86,8 @@ win_error_expand! {
     ERROR_NEGATIVE_SEEK,
     /// The directory name is invalid.
     ERROR_DIRECTORY,
+    /// Cannot create a file when that file already exists.
+    ERROR_ALREADY_EXISTS,
 }
 
 impl WinError {