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: https://github.com/nodejs/node/pull/31235


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>