Add .NET Standard 2.1 support

[Exouxas]

For this package to be compatible with Unity, it apparently needs to compile to .NET Standard 2.1.

[Exouxas]

To use .NET Standard 2.1, the project can't use the following:

• Global using directives
• Target-typed object creation

[Exouxas]

Considering the Unity editor might get updated to use .NET 7 or .NET 8 sometime next year, this issue might not need to be completed.