Make GH treat snapshot outputs as generated files (vercel#308)

The snapshot outputs are a little noisy whenever we change runtime files. This will have GH collapse the file by default
when viewing the PR diffs, so that it's easier to focus on the real changes.

https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github

.gitattributes

@@ -0,0 +1 @@
+crates/turbopack/tests/snapshot/*/output/ linguist-generated=true