* update support for uts39 from unicode 15
* follow uts39's recco that it's not necessary to require
idents to be single-script (they call out proglang idents,
reference the new uts55-5). We use a heuristic derived from
the concept of identifier chunks from uts55-5, to allow
idents like foo_bar_baz where each chunk around the _ can be
single-or-highly-restrictive
* provide directional confusability detection, by reversing
spans of direction-changed chars in idents for bidi_skeleton,
see issue #12929
Jonatan Männchen
José Valim
Luc Fueston
Theodor Fiedler
Panagiotis Nezis
Andrea Leopardi