Genericize Datum type

[jamesarosen]

DatumValue is defined as string | number | Date. Packages like Line define a Datum type as

export interface Datum {
    x?: DatumValue | null
    y?: DatumValue | null
    [key: string]: any
}

When a custom layer function receives the ComputedDatum entries, it can't know the exact type of computedDatum.data.x. It would be nice if Datum were generic, like

export type Datum<X extends DatumValue = DatumValue, Y extends DatumValue = DatumValue> {
  x: X | null;
  y: Y | null;
  [key: string]: any
}

That way, I could build a Line<Date, number> that ensures all my internal types line up.