Extract `changelog-check` to separate action

The `changelog-check` workflow helps us ensure that changelogs are updated alongside changes in polyrepos and monorepos.

Currently, when we make use of this workflow in other repos we need to pin to a specific commit ID. This is cumbersome because it means that if we want to make a change to the workflow we now need to go through each repo by hand and update the commit ID.

If we extracted this workflow to an action, we could start using version strings such as `v1`. As long as new changes were backward compatible they would automatically get propagated to all of the repos.

まず、既存の changelog-check workflow を見つけ、他の repository がどのように利用しているかを確認します。これをバージョン管理された GitHub Action に切り出し、v1-style の参照をサポートするよう利用モデルを更新して、changelog のチェックが polyrepo と monorepo の両方で引き続き機能することを検証します。

索引モデルが issue の本文から書いたものです。