patch 8.0.1239: cannot use a lambda for the skip argument to searchpair()

Problem:    Cannot use a lambda for the skip argument to searchpair().
Solution:   Evaluate a partial, funcref and lambda. (LemonBoy, closes #1454,
            closes #2265)

runtime/doc/eval.txt

@@ -6784,6 +6784,7 @@ searchpair({start}, {middle}, {end} [, {flags} [, {skip}
 		When {skip} is omitted or empty, every match is accepted.
 		When evaluating {skip} causes an error the search is aborted
 		and -1 returned.
+		{skip} can be a string, a lambda, a funcref or a partial.
 
 		For {stopline} and {timeout} see |search()|.