With #34176 we now have granular lint rules created for each compiler ErrorCategory. However, we had remnants of our old error severities still in use which makes reporting errors quite clunky. Previously you would need to specify both a category and severity which often ended up being the same.
This PR moves severity definition into our rules which are generated from our categories. For now I decided to defer "upgrading" categories from a simple string to a sum type since we are only using severities to map errors to eslint severity.
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