mark snaps with @generated

[maxheld83]

Since snaps from expect_snapshot_*() are automatically generated, they should be treated differently by tooling.

For example, I wouldn't want to run a linter on them.

There's a (somewhat widely?) used "standard", the @generated marker for this purpose: https://generated.at
The @generated marker is specifically for generated files which should be committed to SCM
(so not .gitignored).

For example, GitHub's super-linter can be set up to ignore all files thus marked.

I've so far made do with my own transformer like so:

https://github.com/dataheld/elf/blob/0fbc0fdbac7db70c0751b00819423fd58ce1f39e/R/tests.R#L1-L18

Happy to add a PR to this effect, if people find this useful.
(The transform arguments docs suggest a different use,
so @generated probably shouldn't happen via that argument.

Needs some care as to how this can be wrapped in different file formats, i.e. for markdown "<!--@generated-->" seemed idiomatic.