Update model.start() type for two-way reactive functions

[sophiasam96]

Update model.start() type ModelFn to accept an object with get/set methods as an alternative to the function, rather than an alternate return type of that function.

set type also should allow return values as per the docs:
https://derbyjs.github.io/derby/models/reactive-functions#two-way-reactive-functions

[ericyhwang]

Thanks for the type fixes! The Racer tests are still in JS, so we don't have any type-sensitive tests set up yet, or that would've caught the typings bug.

I spotted one minor issue with the new typings, where Ins[] shouldn't have the brackets since Ins is already an array type.

While I was at it, I played around with typings a bit to see if I could come up with something stricter for the set() object return type that validates each key-value in the object individually instead of using a broad index signature.

I found a good approach, and went ahead and pushed the change here.

This TS Playground link demonstrates the types.

Relevant demo reproduced here:

modelFn({
    get(in0: number, in1: string, in2: object) {},
    // get(in0: number, in1: string) {},  // Invalid, inputs must be same arity between get and set
    // get(in0: number, in1: boolean, in2: object) {},  // Invalid, inputs must be same types between get and set
    set(out: unknown, in0: number, in1: string, in2: object) {
        switch (out) {
            case 'A': return [42, 'abc', {}];  // Set all 3 inputs
            case 'B': return [42, 'abc'];  // Set inputs 0 and 1
            case 'C': return { 1: '' };  // Set only input at index 1
            case 'D': return { 1: '', 2: {} };  // Set only inputs at 1 and 2
            // case 'F': return ['oops'];  // Invalid, type of first input must be number
            // case 'F': return { 1: -111 };  // Invalid, type of index 1 must be string
        }
        return null;
    },
});