Create ValueListenablesBuilder class

[jacob314]

We have cases where we need to listen on multiple value listenables but creating a stateful widget wouldn't make sense. Multiple nested value listenable builders are inefficient and hard to read. Create a ValueListenablesBuilder that lets you listen on 2 or 3 value listenables with a builder callback that matches the # of value listenables passed to the constructor. Alternately, create a util that builds a ValueListenable from multiple value listenables returning a tuple with the current values.

[kenzieschmoll]

Reopening this issue. We have DualValueListenableBuilder, which is helpful, but there are still cases where we have more than two ValueListenables we want to listen to and build on changes.