Function to convert a KeyboardEvent to key string

[NCC1701M]

The lib should support the function to convert a KeyboardEvent to a key string.

For example:

hotkeys("*", (evt, handler) => {
   console.log("You've pressed: " + handler.toKeyString());
  // Example output: You've pressed: ctrl+a+s
});

[msimic]

I added a similar issue many months ago, but there were no replies:

https://github.com/jaywcjlove/hotkeys/issues/360

User friendliness and customisation suffer because of lack of this feature.