function add(source, destination, transformFn) {
  const sourceCode = source.getCode();
  const destinationCode = destination.getCode();
  if (!(sourceCode in transforms)) {
    transforms[sourceCode] = {};
  }
  transforms[sourceCode][destinationCode] = transformFn;
}
Registers a conversion function to convert coordinates from the source
projection to the destination projection.

@param {import("./Projection.js").default} source Source.
@param {import("./Projection.js").default} destination Destination.
@param {import("../proj.js").TransformFunction} transformFn Transform.