Convert from dojo/compose to TS2.2 class mixins

[maier49]

Enhancement

TS2.2, currently a release candidate, adds support for class mixins. We should be able to leverage these native class mixins to replace dojo/compose.

This should be more familiar for consumers and provide a lower barrier of entry, and will enable stores to evolve with the latest/next enhancements in Typescript.

[maier49]

This is partially blocked by microsoft/TypeScript#13979. We haven't found a way for a mixin to share a generic with a base class.

A potential workaround is to replace these uses of the mixin pattern with a compositional approach.

[maier49]

In the end we could not work around the limitations caused by the above mentioned issue, and modifying TypeScript to support the desired pattern would be a very difficult and extensive change. As a result, the ObservableStore and QueryableStore are now separate base classes, rather than mixins.