Replace Newtonsoft.Json with System.Text.Json

[ttu]

Replace Newtonsoft.Json with System.Text.Json

Known issues

• Performance: System.Text.Json JsonElement and JsonDocument are not mutable
• Features: System.Text.Json has limited serialization and converter cababilities

Progress:

• CommitActionHandler (in progress)
• DataStore (in progress)
• DocumentCollection
• ObjectExtensions
• Test-files

Once all components are verified to work with System.Text.Json, we plan to release the v3 beta version.

Branch: https://github.com/ttu/json-flatfile-datastore/tree/newtonsoft-to-system-text-json