Add exFAT support (#150)

## Summary

* Swap to updated SDCardManager which uses SdFat
* Add exFAT support
  * Swap to using FsFile everywhere
* Use newly exposed `SdMan` macro to get to static instance of
SDCardManager
* Move a bunch of FsHelpers up to SDCardManager

platformio.ini

@@ -21,6 +21,7 @@ build_flags =
   -DARDUINO_USB_CDC_ON_BOOT=1
   -DMINIZ_NO_ZLIB_COMPATIBLE_NAMES=1
   -DEINK_DISPLAY_SINGLE_BUFFER_MODE=1
+  -DDISABLE_FS_H_WARNING=1
 # https://libexpat.github.io/doc/api/latest/#XML_GE
   -DXML_GE=0
   -DXML_CONTEXT_BYTES=1024
@@ -39,6 +40,7 @@ lib_deps =
   BatteryMonitor=symlink://open-x4-sdk/libs/hardware/BatteryMonitor
   InputManager=symlink://open-x4-sdk/libs/hardware/InputManager
   EInkDisplay=symlink://open-x4-sdk/libs/display/EInkDisplay
+  SDCardManager=symlink://open-x4-sdk/libs/hardware/SDCardManager
   ArduinoJson @ 7.4.2
   QRCode @ 0.0.1