Moves TagHelperSpanVisitor to Legacy namespace

Moves the `TagHelperSpanVisitor` 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/TagHelperSpanVisitor.cs renamed to src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Legacy/TagHelperSpanVisitor.cs

@@ -2,13 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Immutable;
-using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Microsoft.AspNetCore.Razor.Language.Syntax;
 using Microsoft.AspNetCore.Razor.PooledObjects;
 
-namespace Microsoft.AspNetCore.Razor.Language;
+namespace Microsoft.AspNetCore.Razor.Language.Legacy;
 
-internal class TagHelperSpanVisitor : SyntaxWalker
+internal sealed class TagHelperSpanVisitor : SyntaxWalker
 {
     private readonly RazorSourceDocument _source;
     private readonly ImmutableArray<TagHelperSpanInternal>.Builder _spans;