Add hibariya as a reviewer of a couple of gems

[hibariya]

Hi. I added myself as a reviewer of a couple of gems as I was suggested here. For starters, I'd like to work on these gems.

[github-actions]

@hibariya Thanks for your contribution!

Please follow the instructions below for each change.
See also: https://github.com/ruby/gem_rbs_collection/blob/main/docs/CONTRIBUTING.md

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• /merge: Merge this PR if CI passes

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activemodel

You changed RBS files for an existing gem.
This gem does not have reviewers. So you can merge this PR immediately if the CI passes.
We recommend you add yourself to the reviewers for this gem.

---

activerecord

You changed RBS files for an existing gem.
This gem does not have reviewers. So you can merge this PR immediately if the CI passes.
We recommend you add yourself to the reviewers for this gem.

---

activesupport

You changed RBS files for an existing gem.
This gem does not have reviewers. So you can merge this PR immediately if the CI passes.
We recommend you add yourself to the reviewers for this gem.

[hibariya]

/merge