Add 'replacedBy' information to the output of 'dart pub outdated --json'

[devoncarew]

Today, dart pub outdated --json includes a flag indicated that a package has been discontinued:

    {
      "package": "test_api",
      "kind": "transitive",
      "isDiscontinued": false,
    ...

However, it doesn't include the optional 'replaced by' info, even if that exists for a package. It would be nice to include a field - replacedBy? discontinuedReplacedBy? - if the package has that info. I don't believe there's a workaround other than scraping the output of dart pub outdated.

https://github.com/dart-lang/pub/blob/master/lib/src/command/outdated.dart#L525