HackStudio+LibDebug: Support stopping a debugged process

In LibDebug this required implementing the Kill debug action, and
in HackStudio this required making the toolbar's stop action stop
the debugger if active.

Userland/DevTools/HackStudio/Debugger/Debugger.h

@@ -60,6 +60,8 @@ public:
 
     Debug::DebugSession* session() { return m_debug_session.ptr(); }
 
+    void stop();
+
     // Thread entry point
     static int start_static();