Add definitions for missing keys

[edgar-bonet]

The header file Keyboard.h provides definitions for many keys that do not map to printable characters, such as KEY_BACKSPACE, KEY_RIGHT_ARROW, KEY_F1, etc. Some keys are missing, though, to fully cover a standard full-size PC keyboard:

• the Menu key
• the cluster at the top right: Print Screen / SysRq, Scroll Lock and Pause / Break
• the 17 keys of the numeric keypad

This pull request adds macro definitions for these missing keys, thus completing the emulation of a full-size keyboard.

[github-actions]

Memory usage change @ bc2864a

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:sam:arduino_due_x_dbg	0 - 0	0.0 - 0.0	N/A	N/A
arduino:samd:mkrzero	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/Serial flash	%	examples/Serial RAM for global variables	%
arduino:avr:leonardo	0	0.0	0	0.0
arduino:sam:arduino_due_x_dbg	0	0.0	N/A	N/A
arduino:samd:mkrzero	0	0.0	0	0.0

Click for full report CSV

Board,examples/Serial<br>flash,%,examples/Serial<br>RAM for global variables,%
arduino:avr:leonardo,0,0.0,0,0.0
arduino:sam:arduino_due_x_dbg,0,0.0,N/A,N/A
arduino:samd:mkrzero,0,0.0,0,0.0