feat(modeling): preserve color for offset and extrude

[platypii]

This PR preserves the color attribute when applying operations like offset and extrude and project.

This seems like a clear improvement for users of JSCAD. If you have a 2D object with color like on the left, would you expect extrusion to preserve the color? Or lose it?

preserve-color.js

```js import * as jscad from '@jscad/modeling' const { colorize } = jscad.colors const { extrudeLinear } = jscad.extrusions const { square } = jscad.primitives const { translate } = jscad.transforms export const main = () => { const obj2d = [ colorize([1, 0, 0], square({ center: [0, -4] })), colorize([0, 1, 0], square({ center: [0, 0] })), colorize([0, 0, 1], square({ center: [0, 4] })), ] return [ translate([-4, 0], obj2d), translate([4, 0, 0], extrudeLinear({}, obj2d)), ] } ```

If someone really want no color, it's easy to remove using colorize. But if you want to preserve color of sub-components through an operation... there's really no good way to do it without this change.

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• [x] Have you followed the guidelines in our Contributing document?
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[platypii]

@z3dev I just pushed some more changes which I hope will address your concerns:

• returns new geometries even if the shape is the same
• handles empty geometries when it makes sense
• added tests for the above
• addressed other PR feedback (function and variable names)

Let me know what you think