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/dashboard/helper/scriptHelpers.js

@@ -1,5 +1,6 @@
 import AnalyticsHelper from './AnalyticsHelper';
 import DashboardAudioNotificationHelper from './AudioAlerts/DashboardAudioNotificationHelper';
+import { emitter } from 'shared/helpers/mitt';
 
 export const CHATWOOT_SET_USER = 'CHATWOOT_SET_USER';
 export const CHATWOOT_RESET = 'CHATWOOT_RESET';
@@ -8,7 +9,7 @@ export const ANALYTICS_IDENTITY = 'ANALYTICS_IDENTITY';
 export const ANALYTICS_RESET = 'ANALYTICS_RESET';
 
 export const initializeAnalyticsEvents = () => {
-  window.bus.$on(ANALYTICS_IDENTITY, ({ user }) => {
+  emitter.on(ANALYTICS_IDENTITY, ({ user }) => {
     AnalyticsHelper.identify(user);
   });
 };
@@ -34,12 +35,12 @@ const initializeAudioAlerts = user => {
 };
 
 export const initializeChatwootEvents = () => {
-  window.bus.$on(CHATWOOT_RESET, () => {
+  emitter.on(CHATWOOT_RESET, () => {
     if (window.$chatwoot) {
       window.$chatwoot.reset();
     }
   });
-  window.bus.$on(CHATWOOT_SET_USER, ({ user }) => {
+  emitter.on(CHATWOOT_SET_USER, ({ user }) => {
     if (window.$chatwoot) {
       window.$chatwoot.setUser(user.email, {
         avatar_url: user.avatar_url,