Added circle command but i have questions.

I have added the circle command, but this is my first time contributing to something on github, so please if you see that i have done something wrong, done something in a diffrent way than i should have, change it and let me know. I dont know if i should use ellipseMode(center) or not, or if i should use w,h instead of radius.

Start by reviewing the added circle command, then resolve the questions raised about ellipseMode(center) and whether the command should use width and height instead of a radius. Done means the command's API and implementation choices are confirmed by the project maintainer.

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