Fixes#2848
The proper way to convert a unicode scalar in Swift is to use the
`String` initializer that takes a `UnicodeScalar` as an argument. We
were converting a number to a string before, which is incorrect.
This was recently introduced a few days ago. Unfortunately, this doesn't
work as expected. The "function" modifier is not actually the fn key
but used by macOS to represent a variety of "functional" key presses.
This breaks other bindings such as #2411.
I can't find a source on the internet that reliably tells me how we
can detect fn key presses, but I do find a number of sources that tell
us we can't.
Nit picky change, but the OptionSet class (which NSEvent.ModifierFlags
is an instance of) has an API for incrementally setting values without
needing to create an array.
