Added TagDeletedEvent

refs https://github.com/TryGhost/Arch/issues/60

This will be used to update collections when a tag is deleted. Like the Post
events this should not be in the collections package, instead we should have
these as part of the tags and posts packages. These packages don't exist right
now, so I'm following the existing pattern.

ghost/collections/src/events/TagDeletedEvent.ts

@@ -0,0 +1,15 @@
+export class TagDeletedEvent {
+    id: string;
+    data: {slug: string, id: string};
+    timestamp: Date;
+
+    constructor(data: {slug: string, id: string}, timestamp: Date) {
+        this.id = data.id;
+        this.data = data;
+        this.timestamp = timestamp;
+    }
+
+    static create(data: any, timestamp = new Date()) {
+        return new TagDeletedEvent(data, timestamp);
+    }
+}

ghost/collections/src/index.ts

@@ -4,3 +4,4 @@ export * from './Collection';
 export * from './events/PostDeletedEvent';
 export * from './events/PostAddedEvent';
 export * from './events/PostEditedEvent';
+export * from './events/TagDeletedEvent';

ghost/collections/test/TagDeletedEvent.test.ts

@@ -0,0 +1,13 @@
+import assert from 'assert/strict';
+import {TagDeletedEvent} from '../src';
+
+describe('TagDeletedEvent', function () {
+    it('should create a TagDeletedEvent', function () {
+        const event = TagDeletedEvent.create({id: '1', slug: 'tag-1'});
+
+        const actual = event instanceof TagDeletedEvent;
+        const expected = true;
+
+        assert.equal(actual, expected, 'TagDeletedEvent.create() did not return an instance of TagDeletedEvent');
+    });
+});