Add generator-specific context

[Simn]

This PR adds a lightweight version of Common.context which holds all the data generators need. For now it's only implemented for genjvm, but should be relatively easy to add to other generators.

The main goal is to make my dependency dot graphs look nicer.

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[Simn]

I have also already "ported" our second best generator genhl, which was easy enough.

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Unlike in some other areas, smaller is better.

[Simn]

JS fails with some DCE problems, which isn't a great sign for this endeavor... I can't tell what changed here data-wise but it's likely related to the feature feature.

Edit: never mind, the has_feature I originally ported was simply outdated.

[Simn]

This should be ready now, although macos-14 is still failing for some reason.

I've taken the opportunity to remove some generator-specific deprecation warnings because Haxe 5 is the right time to do that. Other than that we should have cleaner internal dependencies now.

@Aidan63 This might cause you some conflicts with your gencpp work, but it hopefully won't be that bad!