Reallow "2>>&1" as a redirection

Appending to an fd doesn't really make sense, but we allowed the
syntax previously and it was actually used.

It's not too harmful to allow it, so let's just do that again.

For the record: Zsh also allows it, bash doesn't.

Fixes #6614

tests/checks/redirect.fish

@@ -8,6 +8,11 @@ end
 outnerr 0 &| count
 #CHECK: 2
 
+
+outnerr appendfd 2>>&1
+#CHECK: out appendfd
+#CHECK: err appendfd
+
 set -l tmpdir (mktemp -d)
 outnerr overwrite &>$tmpdir/file.txt
 cat $tmpdir/file.txt