Register drupal-rector as Composer-based rector-extension
### Problem/Motivation
Added DrupalSetProvider implementing SetProviderInterface to expose per-minor Drupal deprecation and breaking configs as ComposerTriggeredSets keyed on drupal/core. Rector automatically loads matching sets based on the installed core version.
### Work items
- Commit(s): [65d21ee34b](https://github.com/palantirnet/drupal-rector/commit/65d21ee34b1a)
- Pull request(s): [PR #370](https://github.com/palantirnet/drupal-rector/pull/370)
### Credit
- Implemented by: @bbrala
- Reported by: @markfullmer
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*Filed retroactively as part of the Drupal 12 rector coverage effort. Closed as fixed; credit to be applied to the contributors listed above.*
*AI was used extensively to build this change and to file this issue. See [Dries Buytaert: AI-generated Rector rules for Drupal](https://dri.es/ai-generated-rector-rules-for-drupal) and [Björn Brala: Drupal Rector rules are config, not code. Mostly.](https://www.linkedin.com/pulse/drupal-rector-rules-config-code-mostly-bj%C3%B6rn-brala-vfpwe/).*
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