Strictly type parameter of getWeightedValue

[bbugh]

Currently, getWeightedValue is typed as data: any, but it expects the set to have numeric values. This can result in compile-time validity but runtime errors.

Before PR

RNG.getWeightedValue({ 'monster': 'error!' }) // causes runtime error but compiles fine

After PR

// compile-time error: Type 'string' is not assignable to type 'number'.
RNG.getWeightedValue({ 'monster': 'error!' }) 

// works
RNG.getWeightedValue({ 'monster': 30, 50: 10 }) 

If the index signature looks a little odd, here's why: TypeScript expects index signatures to must be literal string or number, but doesn't allow you to have a string | number union. The type in this PR is a signature overload, so TS knows that if the key is a string or a number, the value must be a number, which covers all of the valid cases in TS/JS.

[ondras]

Thanks!