Add additional TFM so STJ can be optional

[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
The System.Text.Json package becomes conditional so only added for the older frameworks and this will require an Additonal TFM added to the project

Describe alternatives you've considered
Accept the additional dependency

Additional context
n/a