feat(renovate-diff): create a standalone tool to provide a semantic insight into diffs between JSON dependency dumps

As a way to provide a more useful way to understand what's changed between exports of Renovate data - via renovate-graph or our other supported formats, we should introduce a separate cli, renovate-diff.

This could be used to i.e.:

• see what changes are coming in as part of a PR/MR
• summarise changes in git diff outputs for projects that source-track their renovate-graph exports
• wire into CI to i.e. raise warnings when changing important dependencies

It makes sense to be part of dependency-management-data, rather than renovate-graph, as we already have some good understanding of the structure here, and can provide a more convenient output format.

Additionally, producing a Go binary will be more convenient i.e. for CI builds.

This should - at least medium term - also produce a stable JSON response output, which can be used for scripting

At some point, we may also expose this, as part of a Go API.