`flutter widget-preview start --web` will cause the widget preview
scaffold to be run as a Flutter Web application using experimental hot
reload support. This will eventually be the default, with the desktop
environment being put behind a flag for use as a fallback under the
assumption that the desktop environment will be removed in the future.
This also wires up the preview detector to trigger hot reloads when new
previews are detected or previews are removed.
Note: while this change results in lib/generated_preview.dart being
generated and updated, it's not currently referenced by lib/main.dart
and the preview environment will render a black screen.
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Co-authored-by: Andrew Kolos <andrewrkolos@gmail.com>
