Add SimplifiedCSharp

[mh-maker2]

The NuGet package needs to respect a few constraints in order to be listed in the curated list:

• Add a link to the NuGet package: https://www.nuget.org/packages/SourceCraftCommunity.SimplifiedCSharp/
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
• The lowest version added must be the lowest .NETStandard2.0 version available
• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
  • if the package is not compatible with standalone player, please add a comment to a Known issues section to the top level readme.md
• All package dependencies with .NETStandard 2.0 target must be added to the PR (respecting the same rules above)
  • Note that if a future version of the package adds a new dependency, this dependency will have to be added manually as well

Note: The server will be updated only when a new version tag is pushed on the main branch, not necessarily after merging this pull-request.

[mh-maker2]

@bdovaz Hello! In my pull request, I added a library that targets netstandard2.1 instead of the required netstandard2.0.
I wanted to clarify whether such a deviation is acceptable in this case. The fact is that the latest versions of Unity (starting from 2021.3) officially support netstandard2.1. This means that users with current versions of Unity will be able to use this library without problems.