Add width/height properties to web viewer

[jprochazk]

What

We now set the web viewer CSS width/height to some default values, which can be modified by the user. Setting them to empty strings is allowed to disable the default values completely, and allow styling the canvas through other means as before.

This is meant as a mitigation for emilk/egui#4699 - by setting the CSS width/height to something, the canvas width/height attributes won't contribute to the display size of the canvas.

Tested in https://github.com/rerun-io/web-viewer-example by manually linking the package

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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• 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
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[abey79]

These changes lgtm (though obviously the underlying DOM geopolitics are way above my head)