Allow optional `index.html` to deploy Angular localized apps

**Is your feature request related to a problem? Please describe.**

When using Angular's i18n feature, the build result will be like:

```
dist
|
| | en
| | | index.html <-----
| | | ...
| | zh
| | | index.html <-----
| | | ...
```

As you can see, there is no `index.html` at the dist root. So the app cannot be deployed without manually creating one.

**Describe the solution you'd like**
Provide an option to allow the app to be deployed without an `index.html` in the dist root.

**Describe alternatives you've considered**
Otherwise, developers have to manually create one via the `postbuild` script:

```ts
"postbuild": "touch dist//index.html"
```