Provide a `dependabot new-ecosystem` command

[deivid-rodriguez]

We're recently trying to be more agile at providing support for new ecosystems in dependabot-core, and I thought it would be nice for people to be able to bootstrap a new ecosystem more easily and let them get all the boilerplate code needed to implement a new by running a command.

And I thought maybe that could be added to the CLI.

One potential problem is that it would couple the CLI with the particular structure of the dependabot-core repo, which is not ideal.

Opening this issue to hear thoughts & opinions about this!