Preferences v2.1.0

• New device support:
  • Any RP2040-based devices
  • Arduino Nano 33 IoT
  • Arduino MKR1010
  • Arduino MKR VIDOR support
• Implement atomic clear() on LittleFS
• Improve FS backend selection
• Fix compilation warnings
• Allow storing empty strings/byte arrays
• Add functional tests (using pio test)

The release is available on PlatformIO, Arduino Library Manager, and Particle Build