Fix CS1726: exclude InternalsVisibleTo from signed assembly build

Strong-name signed assemblies require a public key in InternalsVisibleTo
declarations. Since Tomlyn.Tests is not signed, guard the attribute with
a TOMLYN_SIGNED conditional defined in Tomlyn.Signed.csproj.

Co-authored-by: Copilot <[email protected]>

Author: xoofx

src/Tomlyn.Signed/Tomlyn.Signed.csproj

@@ -5,6 +5,7 @@
     <AssemblyOriginatorKeyFile>key.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
     <TargetFrameworks>net10.0;net8.0;netstandard2.0</TargetFrameworks>
+    <DefineConstants>$(DefineConstants);TOMLYN_SIGNED</DefineConstants>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(TargetFramework)' == 'net10.0' Or '$(TargetFramework)' == 'net8.0'">

src/Tomlyn/Properties/AssemblyInfo.cs

@@ -1,3 +1,7 @@
 using System.Runtime.CompilerServices;
 
+// InternalsVisibleTo is not compatible with strong-name signed assemblies unless the
+// friend assembly is also signed with a known public key. Exclude it for the signed build.
+#if !TOMLYN_SIGNED
 [assembly: InternalsVisibleTo("Tomlyn.Tests")]
+#endif