Joins a list of strings, inserting a separator between each element.
StringRiffle[{s1, s2, s3, …}]
StringRiffle[{{s11, s12, …}, {s21, s22, …}, …}]
StringRiffle[list, sep]
StringRiffle[list, {"left", "sep", "right"}]
StringRiffle[list, sep1, sep2, …]
StringRiffle[{"a", "b", "c"}]
→ a b cStringRiffle[{"a", "b", "c"}, ", "]
→ a, b, c