Feature: Circle modifier keys

[athyfr]

Is your feature request related to a problem? Please describe.
It can be hard to get circles in the intended places with the current system.

Describe the solution you'd like
When using the circle tool (or the rectangle tool) there can be two additional modifier keys that change the behaviour:

• SHIFT: Lock aspect ratio, so circles are perfect.
• ALT: Use starting mouse position as centre of circle.

Describe existing solutions that have this feature
Excalidraw

Additional context
N/A