external editor applications for macOS: Add Emacs
external editor applications for macOS: Add Emacs

This commit follows the steps to add an external editor for macOS (please see: https://github.com/desktop/desktop/blob/1b30bfb027aa0c522bf2d2c48122ce0d9c11e2e2/docs/technical/editor-integration.md).

```
machine:~ $ defaults read /Applications/Emacs.app/Contents/Info CFBundleIdentifier
org.gnu.Emacs
machine:~ $
```
shiftkey/desktop,desktop/desktop,desktop/desktop,desktop/desktop,desktop/desktop,shiftkey/desktop,shiftkey/desktop,shiftkey/desktop