Windows Native: Add call to sentry_init in the main function at compile time

[bruno-garcia]

Follow up from #527

The Native SDK is currently initialized via C# (after the Unity C# layer runs). It works well to capture crashes after the game boots properly. The one downside is that we don't capture crashes of the engine bootstrapping itself. Our docs are written to indicate it works this way because we built it like that for Android and iOS.

Until this task is completed we need to update the docs to mention the early crash detection is for mobile only.

[bruno-garcia]

#527 (comment)

[bruno-garcia]

PR tackling #622

[vaind]

This is on hold now, @bitsandfoxes can you please add a comment with details/status and move to "blocked" or "backlog" in the project? We may also need to update the priority