Feature/add dts

[bigbigDreamer]

Adds a hard dependency on @types/pubsub-js, so that any user who installs the main lib automatically gets the types.

Package managers do not install devDependencies when you install a library. They install just dependencies (and in the case of NPM 7, peerDependencies as well, although that's a separate topic).

[bigbigDreamer]

@mroderick

[Lewiscowles1986]

This feels like it's going in the opposite direction to the one that I mentally hold between library and support-library.

I Could definitely see an argument for the @types/pubsub-js being bundled in a [separate] meta-package, or itself at least suggesting use of PubSubJS.

This patch, feels like it would at-least conceptually make this library dependent on some user-supplied types (which may or may not be accurate) and are not under control of the author(s) of this library (support-library).

Are there other examples of npm libraries that bundle @types? I know Jest at least seems to require separate npm i for it's types package in all the repos we maintain in my job. Open to learning on this, but thought I'd share thoughts.