fs: use async writeFile in FileHandle#appendFile

When operating on a FileHandle, the file has already been opened with
the flag given as an option to fs.promises.open(). This means defaulting
to 'a' has no effect here, and FileHandle#appendFile turns out to
exactly be an alias of writeFile. This is now explicit, saving a stack
frame, object copy and assignment.

PR-URL: #31235
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Anto Aravinth <[email protected]>

lib/internal/fs/promises.js

@@ -70,7 +70,7 @@ class FileHandle {
   }
 
   appendFile(data, options) {
-    return appendFile(this, data, options);
+    return writeFile(this, data, options);
   }
 
   chmod(mode) {