Regex: Fix handling of non capturing groups (?:...)

We were wrongly keeping the `:` as a literal content of the group

src/regex_impl.cc

@@ -189,7 +189,10 @@ private:
                 {
                     auto c = advance();
                     if (c == ':')
+                    {
+                        ++m_pos;
                         content = disjunction(-1);
+                    }
                     else if (contains("=!<", c))
                     {
                         bool behind = false;
@@ -1005,6 +1008,12 @@ auto test_regex = UnitTest{[]{
         kak_assert(not vm.exec("a]c"));
         kak_assert(vm.exec("abc"));
     }
+
+    {
+        TestVM vm{R"((?:foo)+)"};
+        kak_assert(vm.exec("foofoofoo"));
+        kak_assert(not vm.exec("barbarbar"));
+    }
 }};
 
 }