Custom color schemes

[rosslh]

Is your feature request related to a problem? Please describe.

It would be great if you could make your own color schemes instead of being limited the preset ones.

Describe the solution you'd like

• The user should be able to input a list of colors (at least 2) and a color-space to interpolate through.
• The backend should then use those inputs to generate a gradient to be used for the fractal coloring.

I've come up with a low fidelity diagram showing what this might look like in the sidebar:

To pick the individual colors, we can use the browser's built in color picker. The logic for handling the new inputs should live in MandelbrotControls.ts. New properties will need to be added in types.ts and passed as arguments to the Wasm backend in worker.js. Those arguments will then be used by the Rust code to generate a custom gradient to color the image.

Since this involves both Rust and TypeScript, two people may (or may not) want to pair up on this. Use the comments here to organize the work effort.