Make severity a Rule concern

[vinistock]

First step towards #1000

We want to be able to configure and treat graph level diagnostics the same way as linter level diagnostics, but they are not implemented consistently right now. This is the first PR in a small series to make sure that the representations of Rule, Diagnostic and RelatedInformation are all uniform across the codebase, so that we can truly treat them similarly.

This PR does 2 primary things:

1. On the Rust side, we made severity a diagnostic concern, when it should really be a Rule concern. You configure the severity you desire for a class of mistakes, not for the individual occurrences of the mistake
2. We were using kebab case for the unique rule names, but that doesn't match the unique name used for linter rules. It's easier if we standardize on camel case, so that we have consistent naming