Add Net standard 2.1 as TFM

[thompson-tomo]

Is your feature request related to a problem? Please describe.

I want to minimise dependencies in my project by utilising framework dependencies wherever possible

Describe the solution you'd like

I want the package to not have an explicit dependency on dependencies which can be provided by the framework ie System.Reflection.TypeExtensions, System.ValueTuple

Describe alternatives you've considered

Accept the additional dependency

Additional context

No response