TermDebugSendCommand lost it's global visibility when converted to
vim9script. Restore it.
Fixes: 23f29ffc64 ("runtime(termdebug): convert termdebug plugin to
Vim9 script")
closes: #14997
Signed-off-by: Damien Riegel <damien@riegel.io>
Signed-off-by: Christian Brabandt <cb@256bit.org>