Make `bin/dev` available (but not necessary) to run `solid_queue`

[nimmolo]

This makes the Rails 7 standard script ./bin/dev available, and adds a Procfile.dev with one task that will start solid_queue in dev.

[!NOTE] Unlike the Rails 7 default behavior, this MO bin/dev does not start the dev server, because running the server via foreman makes debug unusable. So if someone wanted to start solid_queue locally, they could call rails s in one window, and bin/dev in another.

Admittedly, typing bundle exec rake solid_queue:start is not that much harder than typing ./bin/dev. But the Procfile can potentially have a list of tasks that run when the dev server starts. In the future this is likely what we will want to use to build the SCSS, start redis, action_cable, etc.

The idea of the PR is to make bin/dev an option, and simplify my current branch switching, where these files keep disappearing.

[coveralls]

coverage: 94.446%. remained the same when pulling 7695437b6c54868d39d184e23d557f55debab0cd on nimmo-bin-dev into 40c4cd3c32c28c1dc6a6da1e6bca28539bcedb30 on main.