Add rolling rainfall accumulator to Serial module for WS85 weather station handling rain1h

[mikechristiansen]

This change to serial.cpp and serial.h corrects the cumultivie rain value "rain" reported by the sensor into rain1h useful for reporting precip to windy.com.
This is my first PR with any project, so welcome feedback!

🤝 Attestations

• [√ ] I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • [√ ] NRF52 Pro Micro DIY with WS85 Serial.

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