Add scalafmt to the build

[wemrysi]

We'd like to start using scalafmt everywhere, so adding it here seems reasonable. We can start with the rules from precog/precog and iterate from there.

The ci command should be updated to check formatting and fail the build if not adherent, similar to checking for headers.

[alissapajer]

Will this require adding scalafmt to all of our existing (v2) repos? Or could we opt-in just the new repos and repos we know we're using for v3? I want to avoid spending time reformatting existing repos, if this turns out to be hard for some reason we have yet to discover.

[wemrysi]

No, it shouldn't. I'd prefer if the main branch here can progress forward for new things (clear out stuff we don't need, add scalafmt, switch to 2.13, etc) and we branch if we need to backport something to the existing usages. I'm not sure if trickle is enabled for this, if so we probably want to go around and remove it from repos to avoid the bumps.

[rintcius]

Will also drop Java 8 support as part of this issue.

[rintcius]

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[rintcius]

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