Project: svg-solar
Task (challenging): Add a comet button (id `comet`) in the config panel. Keep it clickable only in `star-planets` view. Click it to request comet name by prompt dialog and generate a comet (class `comet`) that follows an elliptical orbit around the Sun, with its aphelion (farthest point) reaching near Neptune's orbit and its perihelion (closest point) near Mercury's orbit. Comet data format is `{type: 'comet', name: 'halley', r: 0.3, rx: 70, ry: 2, dur: 300, color: 'red', bodies: []}`, rx range is [60, 80], ry range is [2, 4], dur range is [200, 400].