Migrate comment.js Vuex store to Pinia

[theoholl]

• Related to: Migrate to Vue 3 #6663
• Target version: main

Summary

This PR migrated the comment.js Vuex store to Pinia, following patterns from #7790.

[grnd-alt]

did you test this? this breaks reactivity when deleting comments and when loading more comments than on initial fetch

[theoholl]

Oh shoot, I'll review that. I thought I had tested deleting (adding, referencing, etc.) comments at least but I'll put this PR on hold until I tested more thoroughly.

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