Closes#16624
Fixes two issues with aligning the start position and end position to
codepoints when calculating the start and end range.
When aligning the start position:
* use aligned byte index to calculate character index rather than
the unadjusted byte
When aligning the end position:
* do not adjust the end byte if it falls on a UTF-8 codepoint
* align byte to the first byte of the next codepoint rather than the
last byte of the current codepoint
* compute character character end range on the aligned byte index
This commit also adds additional test coverage, including multibyte operations
that previously failed before this commit.
36c401db24
