Allow streaming to the viewer from the cell where it's created

[jleibs]

What

Previously the viewer would not be initialized until the end of cell execution, creating an awkward 2-step process.

Some notes in the anywidget discord got me enough info to hypothesize that something like https://github.com/Kirill888/jupyter-ui-poll might exist... and it works!

By default Viewer.display will now call the new helper that blocks until initialization is complete.

I also decided that display=True on the widget constructor was a little bit odd and changed this to just use Viewer.display().

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
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[github-actions]

Deployed docs

Commit	Link
e8a1c6f	https://landing-58lse3goq-rerun.vercel.app/docs

[jprochazk]

Nice!