Use readonly in params

[mchevestrier]

Since array arguments are not mutated, we can add readonly to array parameter types.

This way we can directly pass readonly arrays to CheapRuler methods:

const ruler = new CheapRuler(42, 'miles');

// Readonly arrays
const a = [1, 2] as const;
const b = [3, 4] as const;

ruler.distance(a, b); // Ok!

Otherwise, we have to needlessly copy the arrays to avoid TypeScript errors:

// @ts-expect-error - Argument of type 'readonly [1, 2]' is not assignable to parameter of type '[number, number]'. The type 'readonly [1, 2]' is 'readonly' and cannot be assigned to the mutable type '[number, number]'.
ruler.distance(a, b);

// or copy the arrays to avoid errors:
ruler.distance([...a], [...b]);