support: Add theme plugin validator

[yuki-takei]

Tasks

• https://redmine.weseek.co.jp/issues/146728

[changeset-bot]

🦋 Changeset detected

Latest commit: ea12546

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@growi/pluginkit	Minor
@growi/core	Minor

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[reg-suit]

reg-suit detected visual differences.

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