The existing bracket_flash only triggered when the cursor was
ON a bracket character. This meant that after typing a closing
bracket (the most common trigger scenario), the cursor sits
AFTER the bracket and the flash doesn't fire.
Update update_bracket_flash to also check the character BEFORE
the cursor (pos - 1) and use it as a fallback. The pair position
is adjusted accordingly (the match is at pos - 1 for the before-
cursor case).
Tests: 1393 pass, zero warnings.
vasilito
daa126cc07