patch 9.1.1187: matchparen plugin wrong highlights shell case statement

patch 9.1.1187: matchparen plugin wrong highlights shell case statement

Problem:  matchparen plugin wrong highlights shell case statement
          (Swudu Susuwu)
Solution: return early, if we are in a shSnglCase syntax element

The shell syntax element "case $var in foobar)" uses closing parenthesis
but there is no corresponding opening parenthesis for that syntax
element. However matchparen is not aware of such things and will happily
try to match just the next opening parenthesis.

So let's just add a way to opt out for such cases. In this case, use the
syntax state to check if the closing parenthesis belongs to the syntax
item "shSnglCase" and if it is, do not try to find a corresponding
opening parenthesis.

Since inspecting the syntax state might be expensive, put the whole
check behind a filetype test, so that matchparen will only perform this
particular check, when it knows the current buffer is a "sh" filetype.

fixes: #16801
closes: #16831

Signed-off-by: Christian Brabandt <cb@256bit.org>