Fix single-file publishing for the CLI 🐛☠️

With the previous version, got this error when trying to run elm-fullstack.exe:
----
Error:
  An assembly specified in the application dependencies manifest (elm-fullstack.deps.json) was not found:
    package: 'LibGit2Sharp.NativeBinaries', version: '2.0.306'
    path: 'runtimes/win-x64/native/git2-106a5f2.pdb'

For context, see:

+ libgit2/libgit2sharp#1754 (comment)
+ dotnet/sdk#3685

implement/elm-fullstack/elm-fullstack.csproj

@@ -9,6 +9,10 @@
     <FileVersion>2020.0518.0.0</FileVersion>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <IncludeSymbolsInSingleFile>true</IncludeSymbolsInSingleFile>
+  </PropertyGroup>
+
   <ItemGroup>
     <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="2.5.0" />
   </ItemGroup>