Added Two.Arc

[jonobr1]

Resolves this issue: #605

By adding a Two.Arc class to /extras. You can use like so:

import { Arc } from 'two.js/extras/jsm/arc.js';
const x = two.width / 2;
const y = two.height / 2;
const width = 50;
const height = 33;
const startAngle = 0;
const endAngle = Math.PI * 0.5;
const arc = new Arc(x, y, width, height, startAngle, endAngle);
two.add(arc); // Don't forget to add it to a scene!

and it has the same properties as any Two.Path:

arc.fill = 'red';
arc.dashes = [5, 2];
arc.stroke = 'black';
arc.linewidth = 1;
arc.scale = 5;