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/shared/helpers/specs/mitt.spec.js

@@ -0,0 +1,25 @@
+import { emitter } from '../mitt';
+
+describe('emitter', () => {
+  it('should emit and listen to events correctly', () => {
+    const mockCallback = jest.fn();
+
+    // Subscribe to an event
+    emitter.on('event', mockCallback);
+
+    // Emit the event
+    emitter.emit('event', 'data');
+
+    // Expect the callback to be called with the correct data
+    expect(mockCallback).toHaveBeenCalledWith('data');
+
+    // Unsubscribe from the event
+    emitter.off('event', mockCallback);
+
+    // Emit the event again
+    emitter.emit('event', 'data');
+
+    // Expect the callback not to be called again
+    expect(mockCallback).toHaveBeenCalledTimes(1);
+  });
+});