feat(developer): add NFC mode to LDML keyboard debugger

The debugger currently works entirely in NFD. This is different to how the end user apps work, as Keyman works in NFC at runtime. We should probably add an NFC mode to the debugger (and which should be the default), so that the default behavior matches what you see in apps. NFD-mode would be there to help diagnose and understand complex transforms (along with single-step features which would be a bigger parcel of work).