Inserts a string at a given 1-based position.
StringInsert["string", "snew", n]
StringInsert["string", "snew", - n]
StringInsert["string", "snew", {n1, n2, …}]
StringInsert[{s1, s2, …}, "snew", n]
StringInsert["abcd", "X", 2]
→ aXbcdStringInsert["abc", "X", 5]
→
StringInsert::ins: Cannot insert at position 5 in abc.
StringInsert[abc, X, 5]