History api fallback does not handle `index.ejs`

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Hi,
First of all. Thanky you for your job! 🙂

I already spend few hours with unsuccessful configuring of `historyApiFallback`

So I set in `union.config.js`:
```
devServer: {
historyApiFallback: true,
},
```

It look that it works well, but problem is that `history-api-fallback` serve `index.html` as default and not `index.ejs`

so if i rename `index.ejs` to `index.html` and `history-api-fallback` starts to work only for nested path (like `localhost:3300/a/b`) because root still goes to `index.ejs`

so I added to `union.config.js`:
```
templateFilename: 'index.html',
```

Now it looks like that it works but if I go to:
`localhost:3300/something/id/something`

it does not render and load any wigets. (maybe some bad relative paths of `

Start with the union.config.js settings for devServer.historyApiFallback and templateFilename, then reproduce the root and nested URL cases described in the report. Compare index.ejs and index.html behavior, including whether widgets load at nested paths; done means history fallback serves the intended template and the widgets render correctly.

Written by the indexing model from the issue text.