gn format: Add 'outputs' to preferred-multiline lists

For symmetry with 'inputs'.

R=brettw@chromium.org,tfarina@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/780543003

Cr-Original-Commit-Position: refs/heads/master@{#306668}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fb0c4fd675c191bcc214088881d66170a2e828c0