Add ability for `yarn` to replace `bower` executable

[alexlafroscia]

Would it be possible, if the bower executable was not found but yarn was, to use Yarn instead to install the alternate bower dependencies? I'm in the process of replacing npm with yarn for the CI setups in many of my addons, and realized that it would be nice to just forgot that bower exists entirely and use yarn for that too 😬

It seems like Bower support in Yarn is rocky right now, but after it stabilizes I'd be happy to add this support myself.

[nathanhammond]

@alexlafroscia We are investigating Yarn but will make no motion toward that until we have done a significant amount of testing and can be sure that they have a compatible LTS story.

I'm closing this issue for now as it's something we're closely monitoring but are unable to act on at this time. We'll be glad to tap you to help with that effort if it ever becomes time!

[wagenet]

Any updates?

[wagenet]

Looks like it is supported, didn't read enough of the README :)