`eventToHotKey` inconsistent `alt` key behaviour on Mac

[theinterned]

On a Mac, the alt (option) key acts as a modifier. For example, when alt and k are pressed, the serialized event string produced on a Mac is Alt+˚, while on Windows and Linux the serialized string is Alt+k.

• This relates to Add a special Mod modifier that translates to either Meta on Mac or Control on other platforms #66 as it means that Alt can not reliably be used with the "Mod" modifier.
• This relates to eventToHotkeyString inconsistent shift key behaviour on Mac #54 as the fix for that will likely be applicable here as well.