keep chop from splitting a trailing surrogate pair

[alhudz]

StringUtils.chop cuts the last UTF-16 char, so a String ending in a supplementary code point is left holding a lone surrogate.

Repro: chop("x😀"), a String ending in U+1F600.
Expected: "x", dropping the whole trailing code point, the same way chop("a") returns "".
Actual: "x\uD83D", only the low surrogate is removed and a lone high surrogate is left behind (malformed UTF-16).
Cause: the cut is str.substring(0, length - 1) with no surrogate check, unlike the sibling abbreviate/truncate which already guard their cuts with splitsSurrogatePair.
Fix: reuse splitsSurrogatePair(str, length - 1) to drop both halves when the final char is a low surrogate. The \r\n handling and every documented example stay unchanged.

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