Support System.Text.Json

[goofballLogic]

System.Text.Json is now built into .NET Core 3 and installable for .NET Standard 2.0+ (e.g. .NET framework 4.7.2+). Can we support this going foward (either instead of or in addition to Newtonsoft Json)

[goofballLogic]

Proposed design here: #59 (comment)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.