pub tool: support adding a dependency as a dev dependency

[kenzieschmoll]

dart pub add dev:foo will add package foo to dev_dependencies instead of dependencies.

[jakemac53]

note there is also the --dev flag which will implicitly do that for all the listed packages.

[jonasfj]

--dev flag is deprecated, please use the dev: prefix -- I think there was a reason for it being done that way.

[jakemac53]

Maybe we should just add text to the tool description for the various formats and let it figure out passing us a valid argument (basically just take the help text from the add command).