chore: Mark vendor folder as generated

each of the files in it is created by vendoring using
`npm run-script build:[...]`.

This way GitHub does respect these as irrelevant in PR reviews, which
should make them easier.

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

.gitattributes

@@ -0,0 +1 @@
+vendor/**/* linguist-generated=true