This adds flutter create --sample which allows users to execute a command which will create a working sample app from samples embedded in the API docs.
The command looks something like this:
flutter create --sample=chip.DeletableChipAttributes.onDeleted mysample
This creates a custom dartdoc tool that will generate snippet blocks in our API docs that allow the user to copy easily to the clipboard, and will also embed the snippet code into a template to show it in a larger context with an app.
This PR adds the snippet tool, a template, and a couple of HTML skeleton files, one for snippets that are designed to be in an application setting, and one where it simply puts a nice container around existing snippets, making them easier to copy to the clipboard.