Support text background

[dwelle]

trafficstars

The idea is to make text more visible on certain background, but also to highlight text (at least until https://github.com/excalidraw/excalidraw/issues/1126 is implemented).

Initially, we could just support the setting the backgroundColor and go with a good default.

A couple options:

1. Rectangular/round bg depending on edges (which we could support as well when backgroundColor is set)

   

   

2. Some sort of a stroke:

   

   It should probably be easy with Canvas: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/strokeText, but not so much with SVG?

Originally requested by @adamkdean

[knightcube]

Is someone working on this?