This PR rewrites the invisible unicode detection algorithm to more
closely match that of the Monaco editor on the system. It provides a
technique for detecting ambiguous characters and relaxes the detection
of combining marks.
Control characters are in addition detected as invisible in this
implementation whereas they are not on monaco but this is related to
font issues.
Close #19913
Signed-off-by: Andrew Thornton <art27@cantab.net>
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