Engine: support GATs

[W95Psp]

This is imported incorrectly:

trait Foo {
    type T<M>;
}

Open this code snippet in the playground

In import_thir.ml, we probably just skip GATs.

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This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[W95Psp]

Still relevant.

[github-actions[bot]]

This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.