feat(filesystem):Add error-code other

Part of #892

Author: ricochet

proposals/filesystem/wit-0.3.0-draft/types.wit

@@ -265,6 +265,10 @@ interface types {
         text-file-busy,
         /// Cross-device link, similar to `EXDEV` in POSIX.
         cross-device,
+        /// A catch-all for errors not captured by the existing variants.
+        /// Implementations can use this to extend the error type without
+        /// breaking existing code.
+        other,
     }
 
     /// File or memory access pattern advisory information.