Merge Craft 3 version into canonical repo

[michaelrog]

Per latest best-practice, we should merge this Craft 3 version back into the original Walk repo: https://github.com/TopShelfCraft/Walk

We can delete the current (1.x) dev branch, rename the current master to 1.x, and add new master and 3.x.dev branches.

We will also need to deprecate the duplicate Composer package, publish a new one using the canonical repo, and update the Walk entry in the Craft plugin store.

Finally, we should copy issues from this repo into the canonical one, and deprecate/archive this repo.

[michaelrog]

The repos are merged, and the package name is updated. The new package is submitted to Packagist, and the legacy package is marked as "Abandoned".

We still need to move over the existing Issues from this repo to the canonical repo, and get P&T to update our entry in the plugin store.