make Join work with const tuple values

[dwelle]

Make Join type work by passing a type of a const value.

Use case:

const columns = ["id", "name"] as const;  
const toSelectQuery = <T extends readonly string[]>(columns: T): Join<T, ","> => {
  return columns.join(",") as Join<T, ",">;
}
const query = toSelectQuery(columns); // "id,name"

Sadly, when passing arguments as literals you still have to cast as const explicitly:

const query = toSelectQuery(["id", "name"] as const); // "id,name"

That will not be necessary in TS@5.0 by using a const modifier, see playground.