Support for &> and &| as convenience redirections

This adds support for &> and &| syntax, which both redirect stdout, and
also apply a redirection of stderr to stdout.

tests/checks/redirect.fish

@@ -0,0 +1,27 @@
+#RUN: %fish %s
+
+function outnerr
+    command echo out $argv
+    command echo err $argv 1>&2
+end
+
+outnerr 0 &| count
+#CHECK: 2
+
+set -l tmpdir (mktemp -d)
+outnerr overwrite &> $tmpdir/file.txt
+cat $tmpdir/file.txt
+#CHECK: out overwrite
+#CHECK: err overwrite
+
+outnerr append &>> $tmpdir/file.txt
+cat $tmpdir/file.txt
+#CHECK: out overwrite
+#CHECK: err overwrite
+#CHECK: out append
+#CHECK: err append
+
+echo noclobber &>>? $tmpdir/file.txt
+#CHECKERR: {{.*}} The file {{.*}} already exists
+
+rm -Rf $tmpdir