Implement text alignment inside its content box for a text node using box-model.

[zwcloud]

For metrics's box-model inspector, this is temporarily implemented via reusing LayoutGroup in Metrics.LayoutTextInRectCentered: https://github.com/zwcloud/ImGui/blob/7d6f3be88583b0fb7bbc5a1316dcaaa31ec25fd6/src/ImGui/Development/Metrics.cs#L462-L480

This should be somehow inlined to be used elsewhere.