feat(emitter): always make discriminator 1st property

[archerzz]

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[ArcturusZhang]

so if we would like to put discriminator as the first property, we definitely need to do more, because here is an example of this fix is not fixing.
Here we have a case that the base model with discriminator, has another base, and we always write the base properties first, and those properties appear before your discriminator.
Therefore I think it needs more changes to fix this issue.

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