Add command (or template) to bootstrap pub workspace

[parlough]

It'd be nice to have a new template or new command to bootstrap a pub workspace, both for a new workspace or in the current directory.

We'd like to use this in some future Dart and Flutter tutorials to streamline the instructions but I also expect it'd be useful for other developers.

dart create -t workspace <directory> [--packages-directory=<workspace packages root directory>] [--packages=<list of workspace package names>]

1. Add a new workspace template.

• Creates the specified directory if it doesn't exist.
• If the specified directory doesn't have a pubspec.yaml, create it.

2. Add an option to create the directory for packages

• It's long, but perhaps --packages-directory
• Defaults to pkgs/ or packages/ (I'm not sure, but dart-lang uses pkgs/)

3. Add an option to create directories for specified sub packages

• Even though we used it for package configs in the past, I believe --packages would be the best option.
• Creates directories for the packages in the --packages-directory from earlier.
• Adds the packages directories to the workspace map in the pubspec.yaml

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Related: #61238

\cc @sigurdm

[bwilkerson]

I like this idea.

The third piece, creating a package in a workspace, should also (or exclusively?) be available as a separate create sub-command. It should be easy for a user to add a new package to an existing workspace when they discover the need / usefulness of doing so.

[bwilkerson]

I hadn't yet seen #61238.

[FMorschel]

I've created dart-lang/pub#4590