Allow to configure the canvas background color using the VSCode settings

[jonahschueller]

I noticed that the background color, which is used by the canvas is not configurable using the VSCode extensions settings. This causes the background color to reset for each individual whiteboard.
It would be great to be able to persist the preferred color hexcode in the settings of VSCode.

...
"excalidraw.backgroundColor": "#0000"
...

I personally like to use a hexvalue of #0000, which will cause a transparent canvas background which blends nicely with my VSCode theme, as the editor color of my theme is used.

As an alternative there could also be an option for the canvas background color of transparent which mimics the same behaviour.

I think such a small feature would be a great improvement for the VSCode extension!