Support for automatically discovering all package ecosystems in the repo

[AArnott]

Is there an existing issue for this?

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Feature description

There should be a way to enable Dependabot (possibly via the .yml file) to automatically discover any and all package dependencies, no matter the package manager or the subdirectory.
Customizing these or turning them off individually should then be an option. But we shouldn't miss the benefits of Dependabot just because we didn't think to add a section to dependabot.yml when we added our first (new package ecosystem here) project to an existing repo.

e.g. When a repo is primarily .NET and then adds node, or rust, or something else, dependabot will simply not ever create a PR for those ecosystems -- until someone thinks to update dependabot.yml.