Break up the base package

[jasongrout]

We had some pain recently where we updated the base widget manager, but not the base widget classes, which forced widget authors to update their dependency on the base package.

From #2472 (comment)

I think moving forward we can:

• break base into base-widget and base-manager
• make a restricted interface for the manager exposed on a widget that lives in base-widget
• version base-widget and base-manager separately
• not sure where to put the jlab registry token- probably base-widget since that is the only thing we want people to depend on

Then base-manager depends on base-widget, and third-party libraries depend on base-widget, and we have two version numbers to communicate to two separate groups of people.

And @vidartf points out that we probably want to bump the compilation target to es2017 as well.

I think this would be an excellent change for 8.0 (it is a js-only thing, so we could probably do it before 8.0 if we weren't bumping the compilation target)

We could probably even leave the base-widget package just called base, and separate out the manager. That would make @jupyter-widgets/base 3.0, and base-manager depend on base 3.0. That would ease upgrading (custom widget authors would just need to update to ^1 || ^2 || ^3)

[mgeier]

Would this have an influence on ipywidgets.embed.DEFAULT_EMBED_REQUIREJS_URL (see #2284)?
Would there be two "default" URLs?

[vidartf]

@mgeier I don't think that url is currently used directly in any place in the base JS package, so I don't think that should be affected.