Support editorconfig for defining diagnostic severity levels

### Summary

Today we support editorconfig for buildcheck diagnostic configurations - but across the stack users are able to use editorconfig to configure Roslyn analyzer and source generator behaviors as well. In practice, this feels a lot like managing the behavior of MSBuild diagnostics, but users hit a wall when they try to do so. We should feed in editorconfig configuration to the decision-making around how to log a diagnostic.

### Background and Motivation

* Unifying the control plan for diagnostics of all kinds
* Enabling control of this outside of the 'MSBuild bubble'
* Better UX

### Proposed Feature

We should feed in editorconfig configuration to the decision-making around how to log a diagnostic.

### Alternative Designs

_No response_