Investigate enabling implicit usings

Now that we switched to C# 10, it might be a good idea to enable [implicit usings](https://devblogs.microsoft.com/dotnet/welcome-to-csharp-10/#implicit-usings).

Start by reading the linked C# 10 implicit-usings guidance and inspecting the repository's project configuration. Determine which projects should enable the setting and whether existing builds and tests remain unaffected. Done means the intended projects use implicit usings consistently and the relevant validation passes.

Written by the indexing model from the issue text.