Extract rule: template-no-at-ember-render-modifiers

[NullVoxPopuli]

Split from #2371.

[johanrd]

Review from claude: comparing against ember-template-lint no-at-ember-render-modifiers

⚠️ Error message recommends the banned pattern?. The PR says: "Use (did-insert), (did-update), or (will-destroy) from ember-render-helpers instead." —The original gives per-modifier messages pointing to custom modifiers:

Do not use the did-insert modifier. This modifier was intended to ease migration to Octane and not for long-term side-effects. Instead, refactor to use a custom modifier. See
https://github.com/ember-modifier/ember-modifier

Fix: replace the message to match the original's guidance, or at least point to ember-modifier instead of recommending the same pattern back? (or is render-helpers ok while render-modifiers not?)

Detection logic and test coverage are otherwise correct. The docs migration section already correctly references ember-modifier — only the lint message is wrong.