Change spec types to allow ReadonlyArray choices

[beaulac]

In TypeScript, the current type definitions don't allow the following:

const myChoices: ReadonlyArray<string> = ['foo', 'bar'];
const myEnv = cleanEnv(process.env, { MY_ENV_VAR: str({choices: myChoices});

// Error:(...) TS4104: The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'any[]'.

Given that the choices field doesn't need to be modified (only the include method is used currently), it should be allowed to be a ReadonlyArray. Generally in TS, it’s good practice to use ReadonlyArray over Array when no mutation is intended.

Any Array<T> is also assignable to ReadonlyArray<T>, so this change doesn't break any existing behaviour.

This change uses ReadonlyArray<T> instead of readonly T[] to maintain compatibility with TS <3.x.

[af]

Looks good to me, thanks!