Add masks field as array of strings

[NivaldoFarias]

I noticed that the mask field's returned value from objects varied between a single string and an array of strings. Since the type definitions declares the field as string only, I added the masks field to return an array of strings and changed the objects that had the mask field return an array to return the first string in the array.

changes:

• adds masks field to objects;
• now, the mask field returns a string only;
• update type declarations with JSDoc;
• update README.md;

I created a temporary file in the root of the project to execute the script below to change JSON file:

import Bun from "bun";

import countries from "./src/countries.json"; 

const updatedCountries = countries.map((country) => {
  const masks = Array.isArray(country.mask) 
    ? country.mask 
    : [country.mask];

  return {
    ...country,
    masks,
    mask: masks[0],
  };
});

await Bun.write("./src/countries.json", JSON.stringify(updatedCountries));

node 22.9.0, bun 1.1.31

[ChristoPy]

Nice addition! It gets a more predictable behavior.

Gonna work this this PR after the next release