fix indexOfDifference splitting a surrogate pair

[alhudz]

Repro: getCommonPrefix("𐐀", "𐐁") returns the lone high surrogate \uD801, and difference("𐐀", "𐐁") returns the lone low surrogate \uDC01. Both are malformed UTF-16.
Cause: both indexOfDifference overloads walk the inputs one char at a time, so when two strings share the high surrogate of a supplementary code point but differ in the low half, the reported index lands inside the pair (index 1 rather than 0). getCommonPrefix then slices the pair in half and difference starts its result on the orphaned low half.
Fix: when the difference falls on a low surrogate whose preceding (common) char is a high surrogate, report the start of the pair. getCommonPrefix and difference follow for free. BMP inputs never hit the new branch, so existing behaviour is unchanged.

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