Improve label-defining component registration ergonomics

Eventually I would like to see the registration as a derive macro, similar to how hooks are registered.

_Originally posted by @viridia in https://github.com/bevyengine/bevy/pull/25007#pullrequestreview-4711131057_

This requires some careful thought in terms of code-organization. Doing so would require us to move the label resolution code deep into the dependency tree, probably in bevy_ecs. That's probably a fine move given the value of having everything declare their own priorities ergonomically, but it should be feature-gated.

We could even add this to the `Component` derive (feature-gated please!) if we did so, sparing us from another type-registry type of pass.

Start by reading the label registration discussion referenced from pull request #25007 and inspect the existing registration and label-resolution organization around bevy_ecs. Compare the proposed derive-macro and Component-derive approaches, including feature-gating; done would require an agreed design and implementation plan, but the issue does not define a concrete completion test.

Written by the indexing model from the issue text.