Make sure that the type discriminator is the first property

[flobernd]

This changes allows to speed up deserialization a lot (for statically typed languages) as the polymorhic variants can be determined by peeking a single JSON key-value pair instead of deserializing a complete object into an intermediate representation.

This as well allows to use built-in polymorphic JSON deserialization with several JSON frameworks (like e.g. System.Text.Json in .NET).

Note:
For other polymorhic objects like Body, Value, Variant, the discriminator is already emitted as the first key already.

Edit:
Needs further work to ensure rest of the properties stay sorted. In the best case, safe-stable-stringify would provide an option to customize the sort order.

[l-trotta]

still have to check if we can improve the java generator somehow, but in the meantime this doesn't break anything, so LGTM

[flobernd]

I have to wait for the author of safe-stable-stringify to release version 2.5.0 on NPM, before we can merge this. Using the GH repo works locally, but not in the CI.