chore(developer): consolidate 'extra' field for FILE_KEYBOARD

FILE_KEYBOARD is a structure used in multiple libraries and languages.
It is passed around between kmcmplib, kmcmpdll, and kmcomp, with three
separate definitions (kmcmplib/compfile.h, kmcmpdll/compfile.h, and
kmcomp/compile.pas). These duplicate definitions must be maintained for
now because of type issues between the legacy kmcmpdll and kmcmplib,
which means that the definitions must be kept in sync.

Because FILE_KEYBOARD_EXTRA has a C++ class in it, it will be
initialized in CompileKeyboardHandle rather than in the caller, so
FILE_KEYBOARD.extra is now a pointer rather than a struct.

When original kmcomp and kmcmpdll are eventually removed, these
duplications will disappear and some of the memory management and
lifecycle will become easier to manage.