The "report alternate keys" part of the Kitty keyboard protocol will
send an additional codepoint containing the "shifted" version of a
key based on the keyboard layout. This is useful for downstream
applications which set up keybindings based on symbols instead of
exact keys being pressed.
For example, underscore (_) with the Alt modifier is sent as minus (-)
with Alt and Shift modifiers. A terminal will send the underscore
codepoint as an alternate though, and we can use that information and
the presence of the Shift modifier to resolve the symbol. Other
examples are 'A-(' (sent as 'A-S-9') and 'A-)' (sent as 'A-S-0').
This change allows pushing the "report alternate keys" flag and
overwrites the keycode and modifiers for any shifted keys sent by the
terminal.
* Add a function for checking keyboard enhancement support
This follows the Kitty documentation's recommended way to check for
progressive keyboard enhancement: query the flags and then query the
primary device attributes (which is broadly supported). If we receive
only the device attributes, the protocol is not supported.
* Check keyboard enhancement in the event-read example
