Regex: add a unit test for why lookaheads dont count for start chars anymore

src/regex_impl.cc

@@ -1264,6 +1264,12 @@ auto test_regex = UnitTest{[]{
         kak_assert(vm.exec("foo", RegexExecFlags::Search));
     }
 
+    {
+        TestVM<> vm{R"(.?(?=foo))"};
+        kak_assert(vm.exec("afoo", RegexExecFlags::Search));
+        kak_assert(*vm.captures()[0] == 'a');
+    }
+
     {
         TestVM<> vm{R"((?i)(?=Foo))"};
         kak_assert(vm.exec("fOO", RegexExecFlags::Search));