(Typescript) Allow ReadonlyArrays as valid classNames parameters

[BenGearset]

Simple reproduction:

const array = ['a', 'b', 'c'] as const;
classNames(array);

Error:

TS2345: Argument of type '[readonly ["a", "b", "c"]]' is not assignable to parameter of type 'ArgumentArray'.   Type 'readonly ["a", "b", "c"]' is not assignable to type 'Argument'.     Type 'readonly ["a", "b", "c"]' is not assignable to type 'Mapping'.       Index signature for type 'string' is missing in type 'readonly ["a", "b", "c"]'.

This PR adds a ReadonlyArray<Argument> to the Argument union type allowing ReadonlyArrays to be used with classNames

[dcousens]

Thanks @BenGearset