Use same severity levels as Roslyn analyzers

The [Roslyn severity levels are](https://learn.microsoft.com/en-us/visualstudio/code-quality/roslyn-analyzers-overview?view=vs-2022#severity-levels-of-analyzers) (showing the `.editorconfig` values below):

* `error`
* `warning`
* `suggestion` ("Info" in VS)
* `silent` ("Hidden in VS; as far as I know, this will show quick-fixes in the IDE, but hide the squiggles/message)
* `none` (completely disabled, including quick-fixes)
* `default` (the default set by the analyzer for that rule)

We could consider using the same values. Currently, we only have `Error`, `Warning`, and `Info` (which could be renamed to "Suggestion"; I find that a more descriptive name, and it fits the Roslyn editorconfig values).

Note that `Default` of course is just for configuration; analyzers should not be able to set this.