LibCore+Everywhere: Remove ArgsParser::add*(char const*&)

This is not guaranteed to always work correctly as ArgsParser deals in
StringViews and might have a non-properly-null-terminated string as a
value. As a bonus, using StringView (and DeprecatedString where
necessary) leads to nicer looking code too :^)

Tests/LibJS/test-test262.cpp

@@ -305,7 +305,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     StringView per_file_location;
     StringView pass_through_parameters;
     StringView runner_command = "test262-runner"sv;
-    char const* test_directory = nullptr;
+    StringView test_directory;
     bool dont_print_progress = false;
     bool dont_disable_core_dump = false;