Feature request: add option to serve in SPA mode

Currently, there is no way to run the app in SPA mode. I would like all requests to go to the root `index.html` where my routing would be handled by the dart code itself using something like [deact](https://pub.dev/packages/deact).

Example:
```bash
webdev serve --auto restart --spa web/index.html
```

Start at the webdev serve command and trace how request paths are currently handled. Compare that behavior with the proposed `--spa web/index.html` invocation, then verify that unmatched requests reach the root index.html while existing serving behavior remains unchanged.

Written by the indexing model from the issue text.