Generating Source package for System.Text.Json (dotnet/corefx#34552)

Generating Source package for System.Text.Json (dotnet/corefx#34552)

* Generating Source package for System.Text.Json

* Changing the placeholder file to be in netcoreapp3.0 instead

* Renaming package

* Updating arcade to get new SourceRewriter package

* Fix package location of resource files

* Adding .resx file to the package and addressing PR Feedback

* Adding description and fixing issue with spacing

* Automatically generate targets file that goes in the package

Commit migrated from https://github.com/dotnet/corefx/commit/09b8b4b1b6c07a7e14385507eaa866e7da83395b