Idea: offer to `yarn install` for dev_cmds if necessary when using devcmd launcher

[lehnerpat]

I just set up devcmd for use in a new project, and a fellow developer (@davidpomerenke) suggested the following:
it would be nice if the devcmd launcher would suggest to directly do yarn install in the dev_cmds dir if necessary.

There are a lot of detailed questions about how to determine when to offer this, and where/how to run yarn install if we do, but from a (first-time) user's point of view, I found this a valid feature request.