runtime(haskell): allow spaces in backticked operators in syntax script

runtime(haskell): allow spaces in backticked operators in syntax script

This formatting (although rare) is actually accepted by GHC, but vim
does not highlight it. This patch adds the simplest possible regex to
support the behavior.

Inconveniently, this might trigger weird formatting on lines that
contain errors, e.g. if the first backtick is removed from:

    a `b` c `d` e

then `c` is going to be marked as an operator, which seems weird but is
valid.

closes: #18776

Signed-off-by: Mirek Kratochvil <exa.exa@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>