Add support for JSON serialization in .net

[Danielku15]

As an alphaTab integrator I want to have a data exchange format between frontends and backends using alphaTab.
Currently the JSON serializer is a web-only component which is used to communicate between the browser main thread and web worker. In some constellations different flavors of alphaTab might need to talk to each other.

e.g.

• a Unity or Blazor Frontend talks to a Node.js Backend.
• a ASP.net backend talks to a JavaScript frontend.

For these usecases it would be good to have the JSON serialization code available on all platforms.

For .net it might be best to only provide the raw object or dynamic but not the string serialization. This way we do not need to pull a dependency like Newtonsoft.Json or System.Text.Json into alphaTab.

[Danielku15]

Feature is now available in version 1.2.0.alpha.71

https://www.alphatab.net/docs/guides/lowlevel-apis/#serialize-data-model-fromto-json