feat: Support fn key as an additional modifier key

[christopherthielen]

I added support for fn as a new modifier key. I tested this locally and it's working great for me. Closes #86

[christopherthielen]

Note to future self:

When running locally, I had problems with the Accessibility prompt/check. The app quits immediately, the system prompts for permission (System Preferences -> Privacy and Security -> Accessibility), but it appears that permission is already granted.

To get the app permissions to stick, I had to:

• Move all copies of Easy Move+Resize to the trash
• Change the Bundle Identifier in XCode project settings -> Targets -> Signing and Capabilities (appended a random number).
• Clean the project artifacts
• After that, I granted permissions to the debug Easy Move+Resize.app and the permissions "stuck"
• I also signed XCode into my apple account (for signing?) but I'm not sure if that's relevant.

[dmarcotte]

Thanks so much, @christopherthielen! Both looks and works great. Will cut a release that includes this change very soon.

And thanks for your "Note to self" here—really helps. I'll update the readme to give some tips around this.