Use platform newline for C# elastic syntax facts

[mwiemer-microsoft]

Extracted from #84347 as a small, reviewable slice.\n\nThis updates the C# syntax facts service so its default elastic end-of-line trivia uses the current platform newline instead of hard-coded CRLF. This is one of the lower-level pieces needed before broader code-action/refactoring line-ending preservation work.\n\nValidation:\n- dotnet test src\Workspaces\CSharpTest\Microsoft.CodeAnalysis.CSharp.Workspaces.UnitTests.csproj --filter FullyQualifiedName~CSharpSyntaxFactsServiceTests

Microsoft Reviewers: Open in CodeFlow

[JoeRobich]

Should we update VB similarly.

[JoeRobich]

I would prefer you rename this to ElasticNewLine or similar.

[CyrusNajmabadi]

This change is not correct and violates the documentation of ISyntaxFacts.

Do not do this:

public SyntaxTrivia ElasticCarriageReturnLineFeed
        => SyntaxFactory.ElasticCarriageReturnLineFeed;
        => SyntaxFactory.ElasticEndOfLine(Environment.NewLine);

The point if the property is to give you the elastic CRLF for VB or C#. It is not too give you an EOL. Let alone an ENV EOL.

This also violates many Roslyn design envariants (like being isolated from the surrounding environment).