Add TextWriter.CreateBroadcasting (#96732)

* Add TextWriter.CreateBroadcasting

* Address PR feedback

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -4,7 +4,6 @@
     <SharedGUID>c5ed3c1d-b572-46f1-8f96-522a85ce1179</SharedGUID>
     <StringResourcesName Condition="'$(StringResourcesName)' == ''">System.Private.CoreLib.Strings</StringResourcesName>
     <GenerateResourcesSubstitutions>true</GenerateResourcesSubstitutions>
-
     <!-- CA2252: Opt in to preview features before using them (for System.Threading.Lock) -->
     <EnablePreviewFeatures>true</EnablePreviewFeatures>
   </PropertyGroup>
@@ -524,6 +523,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\StringWriter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\TextReader.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\TextWriter.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\TextWriter.CreateBroadcasting.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\UnixFileMode.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\UnmanagedMemoryAccessor.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\UnmanagedMemoryStream.cs" />
@@ -2744,4 +2744,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
-</Project>
+</Project>