Moves ClassifiedSpanVisitor to Legacy namespace

Moves the `ClassifiedSpanVisitor` to the `Legacy` namespace since it is only accessed by code in that namespace.

This change is part of an effort to better organize the Razor compiler's internal structure. It specifically isolates legacy code.

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/ClassifiedSpanVisitor.cs renamed to src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Legacy/ClassifiedSpanVisitor.cs

@@ -3,14 +3,13 @@
 
 using System.Collections.Immutable;
 using System.Diagnostics;
-using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Microsoft.AspNetCore.Razor.Language.Syntax;
 using Microsoft.AspNetCore.Razor.PooledObjects;
 using Microsoft.Extensions.ObjectPool;
 
-namespace Microsoft.AspNetCore.Razor.Language;
+namespace Microsoft.AspNetCore.Razor.Language.Legacy;
 
-internal class ClassifiedSpanVisitor : SyntaxWalker
+internal sealed class ClassifiedSpanVisitor : SyntaxWalker
 {
     private static readonly ObjectPool<ImmutableArray<ClassifiedSpanInternal>.Builder> Pool = DefaultPool.Create(Policy.Instance, size: 5);