feat: add support for seesaw encoders

Adds the necessary function addresses for reading and writing encoders on a seesaw.
Also provides two helper functions to make this easier.

examples/seesaw/soil-sensor/main.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"machine"
+	"strconv"
+	"time"
+
+	"tinygo.org/x/drivers/seesaw"
+)
+
+const readDelay = time.Microsecond * 3000
+
+// example reading soil moisture with an Adafruit capacitive soil-sensor (4026) powered by a seesaw
+// https://learn.adafruit.com/adafruit-stemma-soil-sensor-i2c-capacitive-moisture-sensor/overview
+func main() {
+	machine.I2C0.Configure(machine.I2CConfig{})
+
+	dev := seesaw.New(machine.I2C0)
+
+	dev.Address = 0x36
+
+	// the soil sensor is especially slow, let's give it some more time
+	dev.ReadDelay = readDelay
+
+	var buf [2]byte
+	err := dev.Read(seesaw.ModuleTouchBase, seesaw.FunctionTouchChannelOffset, buf[:])
+	if err != nil {
+		panic(err)
+	}
+	moisture := uint16(buf[0])<<8 | uint16(buf[1])
+
+	println("soil moisture: " + strconv.Itoa(int(moisture)))
+}