Reworked translation mapping between Characteristic and Exposed values.

- Broke the code out into its own file.

- Added chaining and flipped translators to assist with re-using
  existing translators.  Also simplified logic when translating between
  HomeKit & Z2M values by removing special cases like the "binary"
  exposes.  Since everything can be expressed with translators &
  translator chains now, the process is streamlined.

- Wired up the defaultTranslator during the setup phase, so when mapping
  is called, there's no nil checks necessary; just a direct call to the
  mapping.Translator.

- Also added more documentation for the translation part since I forgot
  most of it after a year.

z2m_test.go

@@ -152,6 +152,7 @@ func TestMappingTranslation(t *testing.T) {
 		&BoolTranslator{
 			characteristic.ContactSensorStateContactDetected,
 			characteristic.ContactSensorStateContactNotDetected}}
+	initExposeMappings(m)
 
 	for _, test := range []struct{ e, c any }{
 		{"CONTACT", characteristic.ContactSensorStateContactDetected},
@@ -184,6 +185,7 @@ func TestMappingNumeric(t *testing.T) {
 
 	s := service.NewTemperatureSensor()
 	m := &ExposeMapping{exp, s.CurrentTemperature.C, nil}
+	initExposeMappings(m)
 
 	for _, test := range []struct {
 		v    any