Planning for 0.19 migration

Hello fellow maintainers of elm-community!

We are coming up to the time of the Elm lifecycle when we might want to start considering which repos to depreciate, upgrade, or drop.

This cycle is a bit different from the others, as now we have many more repositories than we previously did. Some of them may no longer have a place to fit in with the current ecosystem.

## Deprecation

If you aren't familiar with how deprecation in Elm works, we generally just do not publish a package for a given version. Sometimes users will request a package to be upgraded, in which case it _may_ be worth upgrading. In other cases, it may not be worth it -- entirely dependent on the repo.

## Planned Deprecation

For sure, ones we do not intend to upgrade:
- elm-check

## De-duplicating functions in *-extra

For our *-extra packages, we should aim to identify which functions now exist in core. This includes functions like `List.Extra.andThen`, which is now `List.concatMap`.

## Deprecation or moving current packages

There are others that may be worth depreciating or passing to someone other than elm-community. The goal is for those packages to be re-thought as open problems without solutions.

One such example is `undo-redo`, which makes more sense to exist as an example repo instead of a package, suggesting to users who need this functionality to build it themselves.