Update reserved package name to `data_class`

[felangel]

Apologies for the confusion and inconvenience! The desired package name was meant to be data_class not dart_class.

Related to: https://github.com/dart-lang/pub-dev/pull/7759

• [X] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[felangel]

I thought you wanted dart_class.

But since dataclass is deprecated, I think it's fine to allow data_class to be used.

Though it'd probably be easier to search for a more unique name like coldclass or blueclass, or finalclass or stableclass or immutableclass (hard to spell), orfixedclassorbottledclass` or just stop listing to me rambling and do what you like 🤣

We'll certainly reserve data_class, and in a week or two we can probably create it and transfer ownership to you.

Thanks so much, I really appreciate it! 💙