NintendoSwitch: system keyboard detector and mixed-case typing on FastCodeEntry #157

Part of #141.

BDSP's new-game intro brings its own Switch system keyboard detector and a mixed-case typer (NintendoSwitch_SystemKeyboardDetector, type_on_system_keyboard), buried in the BDSP actions PR (#148). Upstream already types on the same keyboard in FastCodeEntry::keyboard_enter_code (uppercase codes only, blind, QWERTY/AZERTY layouts). Move ours into its own small NintendoSwitch PR that extends FastCodeEntry's layouts with Shift for mixed case and adds the detector and the empty-field check; #148 then calls it.