AK: Update OptionParser::m_arg_index by substracting skipped args

On argument swapping to put positional ones toward the end,
m_arg_index was pointing at "last arg  index" + "skipped args" +
"consumed args" and thus was pointing ahead of the skipped ones.

m_arg_index now points after the current parsed option arguments.

Tests/AK/CMakeLists.txt

@@ -56,6 +56,7 @@ set(AK_TEST_SOURCES
     TestNonnullRefPtr.cpp
     TestNumberFormat.cpp
     TestOptional.cpp
+    TestOptionParser.cpp
     TestOwnPtr.cpp
     TestPrint.cpp
     TestQueue.cpp