This patch applies (mostly) cleanly to the sources included with Arduino 1.8.13 and seems to work well for the purpose, while sporting a simple interface.

I think it should really be considered for inclusion upstream.

Additional context

Additional requests

• https://forum.arduino.cc/t/is-there-a-more-complete-usb-keyboard-library-somewhere/1380932
• LED status (Capslock, Numlock, Scrollock) #6

Related

• Add LED Reporting support to Keyboard emulation. #38
• Adding support for getting numlock/capslock/scrolllock states #43
• Add definitions for missing keys #57