Keep StrBuilder.reverse from splitting surrogate pairs

[alhudz]

Repro: new StrBuilder("a" + "😀" + "b").reverse() returns b\uDE00\uD83Da (a low surrogate ahead of its high surrogate), where new StringBuilder(...).reverse() returns b😀a.
Cause: reverse() swaps the buffer one char at a time, so every surrogate pair is left in low-high order and the supplementary code point becomes malformed UTF-16.
Fix: after the char swap, walk the buffer once and swap each adjacent low-high surrogate pair back to high-low, matching StringBuilder/StringBuffer which this class is documented to mimic. BMP text, lone surrogates and odd-length buffers are untouched.

[garydgregory]

Hello @alhudz , the CI is running... please port to Apache Commons Text as this class is deprecated here. TY!

[alhudz]

Done — ported to Commons Text in apache/commons-text#756, same surrogate-pair restore on TextStringBuilder.reverse.