chore: Replace eventBus with mitt.js [CW-3275] (#9539)

# Replace the deprecated `eventBus` with mitt.js

## Description

Since eventBus and it's respective methods are deprecated and removed
from all future releases of vue, this was blocking us from migrating.
This PR replaces eventBus with
[mitt](https://github.com/developit/mitt). I have created a wrapper
mitt.js to simulate the same old event names so it's backwards
compatible, without making a lot of changes.


Fixes # (issue)

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

1. Made sure all the places we're listening to bus events are working as
expected.
2. Respective specsf or the events from mitt.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

app/javascript/widget/App.vue

@@ -43,6 +43,7 @@ import {
 } from './constants/widgetBusEvents';
 import darkModeMixin from 'widget/mixins/darkModeMixin';
 import { SDK_SET_BUBBLE_VISIBILITY } from '../shared/constants/sharedFrameEvents';
+import { emitter } from 'shared/helpers/mitt';
 
 export default {
   name: 'App',
@@ -157,34 +158,36 @@ export default {
       }
     },
     registerUnreadEvents() {
-      bus.$on(ON_AGENT_MESSAGE_RECEIVED, () => {
+      emitter.on(ON_AGENT_MESSAGE_RECEIVED, () => {
         const { name: routeName } = this.$route;
         if ((this.isWidgetOpen || !this.isIFrame) && routeName === 'messages') {
           this.$store.dispatch('conversation/setUserLastSeen');
         }
         this.setUnreadView();
       });
-      bus.$on(ON_UNREAD_MESSAGE_CLICK, () => {
+      emitter.on(ON_UNREAD_MESSAGE_CLICK, () => {
         this.replaceRoute('messages').then(() => this.unsetUnreadView());
       });
     },
     registerCampaignEvents() {
-      bus.$on(ON_CAMPAIGN_MESSAGE_CLICK, () => {
+      emitter.on(ON_CAMPAIGN_MESSAGE_CLICK, () => {
         if (this.shouldShowPreChatForm) {
           this.replaceRoute('prechat-form');
         } else {
           this.replaceRoute('messages');
-          bus.$emit('execute-campaign', { campaignId: this.activeCampaign.id });
+          emitter.emit('execute-campaign', {
+            campaignId: this.activeCampaign.id,
+          });
         }
         this.unsetUnreadView();
       });
-      bus.$on('execute-campaign', campaignDetails => {
+      emitter.on('execute-campaign', campaignDetails => {
         const { customAttributes, campaignId } = campaignDetails;
         const { websiteToken } = window.chatwootWebChannel;
         this.executeCampaign({ campaignId, websiteToken, customAttributes });
         this.replaceRoute('messages');
       });
-      bus.$on('snooze-campaigns', () => {
+      emitter.on('snooze-campaigns', () => {
         const expireBy = addHours(new Date(), 1);
         this.campaignsSnoozedTill = Number(expireBy);
       });