KetraTermfromPackedOffsets
Creates immutable ranges for displayText from packed UTF-16 offsets.
Return
EMPTY for an empty array, otherwise an immutable validated range set.
Parameters
displayText
text whose offsets the ranges address.
offsets
packed [start0, end0, ...] offsets; the array is copied.
Throws
if offsets are incomplete, empty, unordered, overlapping, outside displayText, or split a surrogate pair.