Let sendSms() name the esm_class messaging mode (#92)

* Regression tests for the messaging mode on sendSms()

* Let sendSms() name the esm_class messaging mode

* Document the messaging mode option and the decision behind it

* Name the constants move for migrating consumers and share the refusal's value naming

* Cover the messaging-mode refusals through submitSms()

* Refuse transaction mode and a report under datagram mode on sendSms()

* Narrow the mode checker and name the default mode once

test/readme.test.ts

@@ -139,6 +139,7 @@ describe('README: Client', () => {
 				flash: false,
 				from: 'MyBrand',
 				message: 'Hello world',
+				messagingMode: 'SMSC_DEFAULT',
 				scheduleDeliveryTime: new Date(Date.now() + 3600_000),
 				to: '46709771337',
 				validityPeriod: 3600,