Add a bigpipe placeholder for the comment form

Problem/Motivation

Inspired by https://www.drupal.org/blog/drupal-10-1-0 → - we don't have a placeholder set up in core yet, but I think we could? Something that looks like one textarea and one button would be a reasonable placeholder for the majority of comment forms I would think.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet