Provide support for webdev configuration yaml file

It would be good if we could introduce a configurable option via a `yaml` file.
Similar to `dart_test.yaml` , we could introduce `webdev.yaml` or `dart_web.yaml`

Example:
```.yaml
serve:
port: 8081
release: true
build:
release: false
output: deploy
```

Start by reviewing the webdev CLI's existing serve and build option handling, then compare the requested structure with dart_test.yaml. Done means agreeing on the configuration filename and supported serve/build fields, implementing their behavior, and verifying that YAML values control port, release, and output settings.

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