Keep TextStringBuilder.reverse from splitting surrogate pairs

[alhudz]

Port of apache/commons-lang#1730 to the Commons Text copy of TextStringBuilder, as requested by @garydgregory.

TextStringBuilder.reverse() swaps the buffer one char at a time, so every surrogate pair is left in low-high order and a supplementary code point becomes malformed UTF-16. StringBuilder/StringBuffer, which this class is documented to mimic, reverse the same input correctly.

Repro: new TextStringBuilder("a😀b").reverse().toString() (a, U+1F600, b).
Before: b\uDE00\uD83Da, a low surrogate ahead of its high surrogate.
After: b😀a, matching new StringBuilder("a😀b").reverse().

Fix: after the char swap, walk the buffer once and swap each adjacent low-high surrogate pair back to high-low, gated on whether any surrogate was seen during the swap. BMP text, lone unpaired surrogates and odd-length buffers are untouched.

Added TextStringBuilderTest#testReverseSurrogatePairs, which fails on the current tree and passes with the fix.

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[garydgregory]

Thank you @alhudz , merged 🚀