📎 Generalise the `DependencyGraph`

[arendjr]

Description

Currently our DependencyGraph only works on JS/TS files. This is done intentionally to allow quick iteration while we still flesh out some of the features and their requirements.

At some point though, we should generalise things and introduce some traits to allow things to work with other file types such as CSS and GraphQL. We'll want to pick up this task as soon as we want to implement a rule with multi-file analysis for one of those languages.