Preserve machine-generated linter rules JSON file

[parlough]

The machine-generated rules.json file found on the gh-pages branch at https://github.com/dart-lang/linter/blob/gh-pages/lints/machine/rules.json is still used by a few clients.

For the purposes of moving to the SDK, it should potentially be checked in to the main branch with a test to make sure it's up to date.

\cc @pq

[pq]

Thinking about a location... We could put it adjacent to all.yaml? (Though example/ is maybe a strange directory for it to live in.) Maybe a doc dir like server has?

@parlough: thoughts?

[parlough]

Yeah I agree that example/ might be a bit confusing since it's not an example.

I was think the generation logic and the file itself could live in /tool/machine to keep things together but a doc directory would be fine too.

[pq]

/tool/machine SGTM. Thanks!