Allow "INVISIBLE ANCHOR" text to be overridden in CSS

Problem/Motivation

Found another PR on the northernco/ckeditor5-anchor-drupal plugin repo that would be a nice-to-have feature.

The "[INVISIBLE ANCHOR: ...]" labels for empty anchors can be a bit jarring/intrusive, particularly when paired with headings.

Allow "INVISIBLE ANCHOR" text to be overridden in CSS by mstrelan · Pull Request #7 · northernco/ckeditor5-anchor-drupal moves the label generation to CSS and doesn't alter current functionality, but allows individual sites to make changes.

Proposed resolution

Merge the PR.

API changes

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Data model changes

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Release notes snippet

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