Improve Severities model

[maridematte]

Context

• Initial version of Analyzers specs #9853 (comment)
• Initial version of Analyzers specs #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

[f-alizada]

Taking this thing to work on since it is aligned with the linked issue.