Add function key support to the keyboard class

[auroJimen]

Currently there's no support of the function key in the keyboard class included on the library, it'd be helpfull to have it in order to use the del, esc & the arrow keys at the same time as the normal keyboard when doing text entry (for instance in a terminal programme or a vim like text editor).

Developers can add it to their programmes manually, but itd be better if it worked from the core M5Cardputer library so as to avoid having a dozen different implementations.