LibCore: Add --version flag to ArgsParser

`--version` always prints "git" for now.

The motivation is that the neofetch port calls `Shell --version` and
adds the output to its output. And if `Shell --version` prints a long
error message about it not knowing the flag, neofetch's output looks a
bit ugly. Per Discord discussion, just add the flag to ArgsParser
instead of only to Shell.

Userland/Libraries/LibCore/ArgsParser.h

@@ -83,6 +83,7 @@ private:
     Vector<Arg> m_positional_args;
 
     bool m_show_help { false };
+    bool m_show_version { false };
     const char* m_general_help { nullptr };
     bool m_stop_on_first_non_option { false };
 };