Plots pane doesn't show thumbnails after a reload

[jmcphers]

To reproduce:

1. Start R
2. Create a few plots (say, 4)
3. Reload the UI or browser

This is the result:

We do correctly re-introduce each plot to the UI after a reload, but we lose the thumbnail in the history filmstrip; it just shows a blank square.

Not necessary for Alpha since that is desktop-only and reloading won't be too common there, but needs to be fixed for acceptable behavior in browser-based versions of Positron.

[jmcphers]

The reason this happens is that there isn't currently such a thing as a thumbnail; we literally use the same image that was initially rendered -- so the only way to get a thumbnail is to generate a full-size image. Generating images can be very expensive, so we don't want to demand that the language runtime churn out a half-dozen or more of them at the same time we are reconnecting and trying to hook up the rest of the UI.

@petetronic pointed out that one way to fix this might be to imitate what we did for the Help pane: have the plots served at an HTTP endpoint, and set the browser headers such that the browser itself could cache the plots and therefore the thumbnails.

Another approach would be to effectively use a screen-capture to copy the plot thumbnail and save the data as base64 encoded PNG in VS Code's storage service. We could do this by drawing the image into a canvas and using toDataURL: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL