Ts file snippet is not rendering

**Steps to reproduce the behavior:**
1. Ember install ember-cli-typescript --save
2. Create a route `docs/button`
3. In `docs/button/controller.ts`

```
// BEGIN-SNIPPET docs-button.ts
@action
onClick(){
}
// END-SNIPPET
```
4. In `docs/button/template.hbs`
`{{demo.snippet name='docs-button.ts'}}`

**Version details**
```
"ember-cli": "~3.8.1",
"ember-source": "~3.8.0",
"ember-cli-addon-docs": "^0.6.7",
"ember-cli-typescript": "^2.0.0",
```

Reproduce the issue using docs/button/controller.ts and docs/button/template.hbs with the listed Ember and ember-cli-addon-docs versions. Start at the demo.snippet invocation and trace how the docs-button.ts TypeScript snippet is discovered and rendered; done means the snippet displays correctly.

Vom Indexierungsmodell aus dem Issue-Text verfasst.