Block library: Refactor Verse block to use class names for text align

[gziolo]

Description

Related: #16027, #15751

This PR follows-up #16035 and fixes #15751.

Currently the verse block get an inline style after selecting a specific alignment.

<pre style="text-align:left" class="wp-block-verse">Content</pre>

This is an unexpected behavior and should work like this:

<pre class="wp-block-verse alignleft">Content</pre>

Without adding the alignment class, I can't set any specific theme side styles for the alignment of verse block.

Before:

<!-- wp:verse {"textAlign":"right"} -->
<pre style="text-align:right" class="wp-block-verse">A <em>verse</em>…<br>And more!</pre>
<!-- /wp:core/verse -->

After:

<!-- wp:verse {"textAlign":"right"} -->
<pre class="wp-block-verse has-text-align-right">A <em>verse</em>…<br>And more!</pre>
<!-- /wp:verse -->

How has this been tested?

Using one of the existing branches (master probably) add a few Verse blocks and set different text alignments and save your post. Open the same post with this branch and ensure that it still works

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[gziolo]

Applying Needs Dev Note as this might have an impact on themes which used the presence of style as a way to change the visual aspect of the block.