[Blazor] JS assets in `Microsoft.AspNetCore.Components.CustomElements` are not getting included

In the `release/9.0` branch, JS assets are not being correctly included in `Microsoft.AspNetCore.Components.CustomElements`. This causes `RegisterCustomElement()` to throw with the following exception message:
```
Error: Could not find 'registerBlazorCustomElement' ('registerBlazorCustomElement' was undefined).
```