Add converse.js as NPM dependency to package.json

Instead of using `git submodules`, you can then use `npm link
converse.js` to link in an existing checkout of converse.js

Note, before running `npm link converse.js` inside the
xmpp-chat-component checkout, you'll need to run `npm link` inside the
converse.js checkout.

Doing it this way removes the need to check out converse.js again
(via git submodule) when you already have a checkout of converse.js
somewhere and also makes the NPM dependency on converse.js explicit.

package-lock.json

+{
+  "name": "chat.component.dev",
+  "version": "1.0.0",
+  "lockfileVersion": 1,
+  "requires": true,
+  "dependencies": {
+    "converse.js": {
+      "version": "3.3.2",
+      "resolved": "https://registry.npmjs.org/converse.js/-/converse.js-3.3.2.tgz",
+      "integrity": "sha512-Kbz+qZgiXaKEINGhUV8WjRRBckEbdQYKXQ5WYEggkLYNaL3dP2XUmE1Y1War0JaQdtsyGm22rIrz/OSy8Y+m+A==",
+      "dev": true
+    }
+  }
+}

package.json

@@ -6,7 +6,9 @@
   "dependencies": {
     "webcomponentsjs": "^1.0.2"
   },
-  "devDependencies": {},
+  "devDependencies": {
+    "converse.js": "^3.3.2"
+  },
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1"
   },