Dependabot-core: Support for Carthage

[FranklinYu]

Carthage is another dependency manager for Cocoa system. Should be easier than dependabot/feedback#221 because we don’t need to clone a giant Git repository.

[feelepxyz]

@FranklinYu thanks for the suggestion! We've been holding off on adding new languages for a while to get all existing languages into a stable state. Still got a way to go so will hold of on this until we get some breathing room ✌️🤠

[stale[bot]]

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