Updates @twilio/cli-core to version 7 (#35)

* Updates @twilio/cli-core to version 7

Switches out @oclif/command and @oclif/config for @oclif/core.
Updates ./bin/run and commands/watch.js to use @oclif/core.
Adds new ./bin/dev script for debug running locally.
Adds src/index.js and main property in package.json

* Removes moment dependency

bin/dev

@@ -0,0 +1,15 @@
+#!/usr/bin/env node
+
+const oclif = require('@oclif/core');
+
+// In dev mode -> use ts-node and dev plugins
+process.env.NODE_ENV = 'development';
+
+// In dev mode, always show stack traces
+oclif.settings.debug = true;
+
+// Configure the Twilio CLI environment
+require('@twilio/cli-core').configureEnv();
+
+// Start the CLI
+oclif.run().then(oclif.flush).catch(oclif.Errors.handle);

bin/dev.cmd

@@ -0,0 +1,3 @@
+@echo off
+
+node "%~dp0\dev" %*

bin/run

@@ -1,9 +1,12 @@
 #!/usr/bin/env node
 // Load up the command module *first* so that we're the parent rather than
 // some other dependency.
-const command = require('@oclif/command');
 
-require('@twilio/cli-core').configureEnv();
-command.run()
-  .then(require('@oclif/command/flush'))
-  .catch(require('@oclif/errors/handle'));
+const oclif = require("@oclif/core");
+
+require("@twilio/cli-core").configureEnv();
+
+oclif
+  .run()
+  .then(require("@oclif/core/flush"))
+  .catch(require("@oclif/core/handle"));