Check caps/num/scroll lock state

[glitchassassin]

Adds lock_state() call for each platform, as well as a module-level call to the API.

[glitchassassin]

I have tested on Windows, Linux, and Mac with the Caps Lock part. Scroll Lock and Num Lock should work the same way with their respective flags, but my main computer is a Mac so I haven't been able to confirm for certain.

To test, just run:

python3 -c "import keyboard; print(keyboard.lock_state('scroll'))"
python3 -c "import keyboard; print(keyboard.lock_state('num'))"
python3 -c "import keyboard; print(keyboard.lock_state('caps'))"

and verify that the output matches the keyboard state.

[glitchassassin]

@boppreh If you had a different function signature in mind for this feature, let me know and I'll update accordingly.