Add support for Rush

[Rush](https://rushjs.io/) is a opensource monorepo manager (similar to Lerna) and is extensively used to manage large monorepos within Microsoft. It'll be great to see if dependabot-core can support Rush monorepos too!

We are currently using a fork of dependabot-core which we have modified to support Rush and would be happy to contribute the changes back.

Creating this issue to start the conversations and see if other people are also interested.

The issue names no files, tests, or entry points. Start by reviewing the Rush documentation and existing support for similar monorepo managers such as Lerna, then compare the reported fork changes with current dependabot-core behavior. Done should include defined tests and a complete Rush monorepo update workflow.

Written by the indexing model from the issue text.