This is the initial tooling work for Flutter Widget Previews, adding two
commands: `flutter widget-preview start` and `flutter widget-preview
clean`.
The `start` command currently only checks to see if
`.dart_tool/widget_preview_scaffold/` exists and creates a new Flutter
project using the widget_preview_scaffold template if one isn't found.
The widget_preview_scaffold template currently only contains some
placeholder files and will be updated to include additional code
required by the scaffold.
The `clean` command simply deletes `.dart_tool/widget_preview_scaffold/`
if it's found.
This change also includes some refactoring of the `create` command in
order to share some project creation logic without requiring `flutter
widget-preview start` to spawn a new process simply to run `flutter
create -t widget_preview .dart_tool/widget_preview_scaffold`.
Related issue: https://github.com/flutter/flutter/issues/115704
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Co-authored-by: Andrew Kolos <andrewrkolos@gmail.com>
