v1.9.0 to add more Timers to SAMD21

### Releases v1.9.0

1. Add TC4, TC5, TCC1 and TCC2 Timers to SAMD21
2. Add example [SAMD21_MultiTimers](examples/SAMD21_MultiTimers) to demo the how to use all 6 SAMD21 timers simultaneously.
3. Add functions `attachInterruptInterval_MS()` and `setInterval_MS()`
4. Rewrite examples to take advantage of new functions and timers

Author: khoih-prog

README.md

@@ -74,7 +74,6 @@
   * [5. Change_Interval on Arduino SAMD21 SAMD_NANO_33_IOT](#5-Change_Interval-on-Arduino-SAMD21-SAMD_NANO_33_IOT)
   * [6. RepeatedAttachInterrupt_uS on SAMD21 SEEED_XIAO_M0](#6-RepeatedAttachInterrupt_uS-on-SAMD21-SEEED_XIAO_M0)
   * [7. SAMD21_MultiTimers on SAMD21 SAMD_NANO_33_IOT](#7-SAMD21_MultiTimers-on-SAMD21-SAMD_NANO_33_IOT)
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 * [Debug](#debug)
 * [Troubleshooting](#troubleshooting)
 * [Issues](#issues)