RFC: less chonky version

# Problem

I love using `wait-on` to wait-on stuff, but the dep stack is _girthy_--300kb+!

# Discussion

Any interest in making a thin variant? rxjs is awesome, but it alone is a fat cat.

Food for thought! Thanks for a great lib! Feel free to close at will

Start by reviewing wait-on's dependency stack and how it uses rxjs, since the issue proposes a thinner variant of the JavaScript command-line utility. Determine a concrete scope for reducing the 300kb+ dependency footprint and define completion by a smaller install without losing the existing wait-on behavior.

Written by the indexing model from the issue text.