Allow adding non-`Level` context to `Annotation`s

[epage]

Currently, an Annotation is created with a Level. We emulate rustc's "context" style by using a Level::Info but this isn't accurate (and adds a prefix).

[epage]

Proposal: Add a new enum with states:

enum Unnamed {
    Message,  // in rustc terms, "primary"
    Context, // in rustc terms, "secondary"
    Level(Level), // new to rustc but maintains our current API
    // Added, // for when we support suggestions
    // Removed, // for when we support suggestions
}

We could then create Annotations off of this

I originally called this Style but this is less of a stole and more of a purpose or role of the annotation. Still unsure what to call it.