Remove unsafe code from String.Join (#44126)

Remove unsafe code from String.Join (#44126)

* Remove unsafe code from String.Join

- Use a span for the separator to share the same code across a char separator and string separator, rather than using pointers.
- Consolidate argument validation for the value/startIndex/count overloads into one shared helper
- Change indexing to use standard 0 to values.Length for loop to eliminate bounds checking on span accesses

* Update src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs