Add Comments block.

[JakePT]

Description of the Change

Adds a Block version of the Comments widget as the "Search Comments" block.

The block replicates the functionality of the Comments widget with the addition of an editable <label> element for the search form.

Closes #2972

How to test the Change

1. Open the blocks-based widgets editor.
2. Click to insert a block, search for the Search Comments (ElasticPress) block.
3. The block should appear and function the same as the legacy Widget.
4. With the Classic Widgets plugin enabled, add the Comments — ElasticPress widget.
5. Disable Classic Widgets and open the blocks-based widgets editor.
6. It should be possible to transform the legacy widget into the new block.

Changelog Entry

• Added: A Search Comments block for searching comments. Replaces the Comments widget in the block editor.

Credits

@JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@JakePT It seems e2e tests are not passing. Do you mind checking them so we can merge this PR? It may be worth giving a look at the solutions we've implemented to the intermittent failures we had for facets here.

[felipeelia]

@JakePT this looks great. Do you mind addressing the minor issues I highlighted before we merge it? Thanks!