[perl #132232] [UNI] ignorecase returns too many graphemes for expanding foldcase graphemes

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# New Ticket Created by  Samantha McVey 
# Please include the string:  [perl #132232]
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`is '_fi_' ~~ /:i fi /, "fi", "ignorecase with ligature needle matches
only ligature needle";`

Returns: 'fi_' instead of just 'fi'. This is because 'fi' expands to 'f' +
'i' under foldcase.