[codegen] add @generated pragma to generated files

[slonoed]

We use Phabricator as a diff tool and code management. It supports // @generated comment on top of the file and marks such file as generated for diffs. Would be cool if codegen add this comment to type files.

[JakeDawkins]

Hey @slonoed! Thanks for opening this. I don't see any issue with adding this. Should it be applied to all file types, not just flow/ts?

[slonoed]

Hi @JakeDawkins

Done in #1748