Reverses the contents of this string builder and returns this instance.
Surrogate pairs included in this string builder are treated as single characters. Therefore, the order of the high-low surrogates is never reversed.
Note that the reverse operation may produce new surrogate pairs that were unpaired low-surrogates and high-surrogates before the operation.
For example, reversing
"\uD800\uDC00" which is a valid surrogate pair.