Improve Severities model

[Mariana Dematte]

Context

• #9853 (comment)
• #9853 (comment)

Goals

• Severities aligned with Roslyn - https://learn.microsoft.com/en-us/visualstudio/code-quality/use-roslyn-analyzers?view=vs-2022#configure-severity-levels, we can omit silent.
• The isEnabledconfiguration is passed to Analyzer Initialize call - and is basically an evaluation of Severity != BuildAnalyzerResultSeverity.None
• Logic properly reflected in the design doc