Roadmap to v1.0

[twavv]

This issue is meant to keep track of what needs to do/has been done to release WebIO v1.0.

• Update to JSExpr v1 (Update for JSExpr v1.0. #356)
• Synchronous observable updates (RFC: Synchronous updates by default #367)
• Add renderable trait (Add renderable trait #361, this might deserve more discussion)
• Improve serialization for IJulia (Don't double/triple serialize WebIO nodes in Jupyter #351)
• Finalize extensible messaging protocol (Reevaluate Julia->browser requests #323)
• Implement scope reaping (Allow scope/observable reaping #321)
• Implement Observable pull-on-mount behavior (Avoid Stale Observable Values #258)
• Remove deprecations
• Remove test dependency on Blink since Blink depends on WebIO (maybe use Electron.jl)

I think there will be some fairly large API changes. I'm not too worried about it because most users of WebIO are really just using Interact/PlotlyJS/MeshCat and we can make sure that WebIO works for those uses cases.

After v1.0, I would want to be relatively hands-off and avoid ever breaking anything ever again. :^) Maybe add a few other features if they're requested but mostly go into maintenance mode.