Add `using fs::SPIFFSConfig` to FS.h (#6324)
Add `using fs::SPIFFSConfig` to FS.h (#6324)

The SPIFFS config object was defined in FS.h in its own namespace, but
is not made easily available like other SPIFFS and FS objects because of
a missing `using` statement.  Add it in FS.h

Fixes #6322
sticilface/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,esp8266/Arduino,esp8266/Arduino,esp8266/Arduino,sticilface/Arduino,sticilface/Arduino,esp8266/Arduino