Pass arguments to webr::canvas() graphics device during captureR()

[georgestagg]

Fixes #390.

@coatless What do you think of this implementation? Here in addition to accepting captureGraphics: true or false, optional arguments can be passed to the graphics device in the following way:

const shelter = await new webR.Shelter();
const capture = await shelter.captureR("hist(rnorm(1000))", {
  captureGraphics: {
    width: 504,
    height: 252,
  }
});

Passing an object here implies capture: true. This should allow for evaluation-level setting of graphics device options without having to start the device manually in the provided R code.

[coatless]

@georgestagg loving the new graphics capture options with built-in backward compatibility.

Thanks for even adding in a check to fetchwebr::canvas()!