configuration: Add auto stub toggle that resets on boot

Auto-stub is an experimental debugging feature that may cause unforseen bugs. This adds a toggle to only allow auto-stubbing unimplemented functions when explicitly enabled when yuzu is launched.

src/core/settings.h

@@ -222,6 +222,7 @@ struct Values {
     bool quest_flag;
     bool disable_macro_jit;
     bool extended_logging;
+    bool use_auto_stub;
 
     // Miscellaneous
     std::string log_filter;