Add support for System.Text.Json

[eNeRGy164]

As remarked by @YVbakker in dendrodocs/dotnet-tool#19, investigated possibility to add support for System.Text.Json.
On first glance, this seems possible. Will need more test coverage for more confidence.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• Added tests to compare both implementations
• Verified with using the PR build in the Tool

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[coveralls]

Pull Request Test Coverage Report for Build 10855403140

Details

• 126 of 126 (100.0%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+26.1%) to 63.275%

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Totals
Change from base Build 10854971307:	26.1%
Covered Lines:	391
Relevant Lines:	571

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💛 - Coveralls