Allow range to extend past the end of the last line, per LSP spec

[davidwengier]

Fixes the following exception, reported in https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1868418 (though I couldn't repro it 🤷‍♂️)

[Error - 4:03:22 PM] System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. (Parameter 'Range end line 50 matches or exceeds SourceText boundary 50.')
   at Microsoft.AspNetCore.Razor.LanguageServer.Extensions.RangeExtensions.AsRazorTextSpan(Range range, SourceText sourceText) in /_/src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Extensions/RangeExtensions.cs:line 172
   at Microsoft.AspNetCore.Razor.LanguageServer.Semantic.TagHelperSemanticRangeVisitor.VisitAllNodes(RazorCodeDocument razorCodeDocument, Range range, RazorSemanticTokensLegend razorSemanticTokensLegend, Boolean colorCodeBackground) in /_/src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Semantic/Services/TagHelperSemanticRangeVisitor.cs:line 40
   at Microsoft.AspNetCore.Razor.LanguageServer.Semantic.RazorSemanticTokensInfoService.GetSemanticTokensAsync(TextDocumentIdentifier textDocumentIdentifier, Range range, VersionedDocumentContext documentContext, RazorSemanticTokensLegend razorSemanticTokensLegend, Guid correlationId, CancellationToken cancellationToken) in /_/src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Semantic/Services/RazorSemanticTokensInfoService.cs:line 88
   at Microsoft.AspNetCore.Razor.LanguageServer.Semantic.SemanticTokensRangeEndpoint.HandleRequestAsync(SemanticTokensRangeParams request, RazorRequestContext requestContext, CancellationToken cancellationToken) in /_/src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Semantic/SemanticTokensRangeEndpoint.cs:line 71
   at Microsoft.CommonLanguageServerProtocol.Framework.QueueItem`3.StartRequestAsync(TRequestContext context, CancellationToken cancellationToken)
[Error - 4:03:22 PM] Request textDocument/semanticTokens/range failed.
  Message: Specified argument was out of the range of valid values. (Parameter 'Range end line 50 matches or exceeds SourceText boundary 50.')
  Code: -32000