Type Alias JoinStringTypes<Vs, Sep>

JoinStringTypes: Vs extends readonly [
    infer A extends string,
    infer B extends string,
    ...(infer Rest extends string[]),
]
    ? JoinStringTypes<[`${A}${Sep}${B}`, ...Rest], Sep>
    : Vs extends [infer A extends string] ? A : ""

Combine an array of string literals into a single string literal, separated by the provided separator.