Support bitflags version 2

[dzfrias]

Is your feature request related to a problem? Please describe.
Bitflags v2 provides a ton of conveniences for types implementing Bitflags. Currently, both the KeyModifiers and KeyboardEnhancementFlags types use the bitflags! macro. It would be nice to use version 2 so their APIs get access to the new features.

This includes a few breaking changes, so there a couple ways to handle it.

Let me know if you want me to submit a PR for any of the options!

Describe the solution you'd like
Use Bitflags v2 for the KeyModifiers and KeyboardEnhancementFlags types. This would require an increment of the minor version of this crate.

Describe alternatives you've considered if any
Either to not go through with this change or gate it with a feature flag.