Shell: Allow non-exhaustive 'case' statements in POSIX

Author: https://github.com/alimpfard
Commit: 8a042cd9cb
Pull-request: https://github.com/SerenityOS/serenity/pull/18429
Reviewed-by: https://github.com/nico ✅

Userland/Shell/Shell.h

@@ -308,6 +308,8 @@ public:
 
     void notify_child_event();
 
+    bool posix_mode() const { return m_in_posix_mode; }
+
     struct termios termios;
     struct termios default_termios;
     bool was_interrupted { false };