fix: add an napi handle scope for the callback

[MarshallOfSound]

Required for Electron 10 where the memory-leak-handle-scope was removed

cc @deepak1556

[ghost]

All CLA requirements met.

[deepak1556]

@alexdima for context, what @MarshallOfSound is referring to is electron/electron#22531 where electron previously had a global handle scope installed which allowed for memory leaks.

[alexdima]

👍 Thank you!

[alexdima]

published native-keymap@2.1.3

I noticed that this generates a warning:

> node-gyp rebuild

  CXX(target) Release/obj.target/keymapping/src/string_conversion.o
  CXX(target) Release/obj.target/keymapping/src/keymapping.o
  CXX(target) Release/obj.target/keymapping/src/keyboard_mac.o
../src/keyboard_mac.mm:184:21: warning: unused variable 'scope' [-Wunused-variable]
  napi_handle_scope scope;
                    ^
1 warning generated.

That is OK for me if it avoids a crash, but it would be nice if we could avoid the warning :).

[MarshallOfSound]

@alexdima Fixed that warning and actually made this fix work (whoops) over here --> #23