Handle backslashes properly in locate_brackets_of_type

This needs to be rewritten, I'm pretty sure we have like 6 of these
kinds of ad-hoc "is this quoted" things lying around.

But for now, at least don't just check if the *previous* character was
a backslash.

Fixes #7685.

tests/checks/basic.fish

@@ -484,3 +484,7 @@ echo '-n art'
 echo banana
 # CHECK: -n art
 # CHECK: banana
+
+# This used to be a parse error - #7685.
+echo (echo hello\\)
+# CHECK: hello\