Make KnownType class and fields public, to make it usable in SyntaxFacade

[antonioaversa]

Why

The KnownType class, defined in SonarAnalyzer.Common is internal, as well as the long list of its static readonly KnownType fields.
The implementation of #6703 (#6728) required adding a new method to the SyntaxFacade class (c.p. #6728 (comment)):

public abstract bool IsKnownAttribute(SyntaxNode attribute, KnownType knownAttribute, SemanticModel semanticModel);

SyntaxFacade is a public class implemented by CSharpSyntaxFacade, in SonarAnalyzer.CSharp, and by VisualBasicSyntaxFacade in SonarAnalyzer.VisualBasic.

This PR solves the accessibility inconsistency that adding IsKnownAttribute to SyntaxFacade would produce.

The change, originally made in #6703, has been isolated into this PR to keep the git history clear.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication

[andrei-epure-sonarsource]

LGTM, feel free to merge once green

[antonioaversa]

https://peach.sonarsource.com/ is up, and running with the latest analyzers.

The latest Cirrus CI build has correctly analyzed most of the projects: https://cirrus-ci.com/build/6016242381750272

The change made in this PR was a change in accessibility modifiers => validation OK.