slides/whiteboard: implement tool to draw shapes

[williamstein]

It would be very useful to have a tool to at least draw some basic shapes, especially now that we have slides.

This would be another tool that's accessible from the toolbar, with a bunch of presets which can be customized.

The parameters:

• type of shape: square, rectangle, circle, ellipse, etc. Start with just a few.
• border color and thickness
• fill color (and opacity)

Here are some screenshots showing what similar software provides for shapes.

Powerpoint:

Google Slides:

Miro:

(miro has a lot of shapes!)

Figma:

This react library might be a good way to render many of the shapes: https://www.npmjs.com/package/react-awesome-shapes

[mforbes]

One feature that is really nice about Notability is that if you draw a shape and then hold it, the curve will snap to a real shape. Thus, if you draw an approximate line, then hold, the line will become smooth. Likewise for circles, squares etc.