[Heading block] Add a wp-block-heading CSS class (#42122)

## What?
This PR adds the `wp-block-heading` CSS class to the heading block.

It happens server-side so:

* The class couldn't be accidentally removed from the serialized markup
* There's no need to migrate the existing posts

## Why?

Without the classname, global styles can't distinguish between the `h1-h6` elements that belong to the heading block vs ones that don't. As a result, [the styles set on the `core/heading` block of theme.json gets applied to all h1, h2, h3, etc](#42082)

## How?

This PR transforms the stored `heading` block markup into one with an additional class name using  [WP_HTML_Tag_Processor](https://make.wordpress.org/core/2022/08/19/a-new-system-for-simply-and-reliably-updating-html-attributes/). 

## Alternatives considered

I explored leaning on `get_block_wrapper_attributes`, but it falls short:

* It infers attributes from globals.
* It only processes the `style` and `class` properties, meaning a heading block with an anchor set would lose its `id`.
* It ignores any additional attributes inside of the block content.
* We'd still need to some parsing to remove the wrapping block from `$content`.

## Testing Instructions

1. Create a new page
2. Add a few heading blocks
3. Align text to right in one of them
4. Add a custom CSS classname in another
5. Save
6. Visit the page, confirm the rendered headings have the `wp-block-heading` css class

docs/reference-guides/core-blocks.md

@@ -284,7 +284,7 @@ Introduce new sections and organize content to help visitors (and search engines
 
 -	**Name:** core/heading
 -	**Category:** text
--	**Supports:** __unstablePasteTextInline, align (full, wide), anchor, color (background, gradients, link, text), spacing (margin, padding), typography (fontSize, lineHeight), ~~className~~
+-	**Supports:** __unstablePasteTextInline, align (full, wide), anchor, className, color (background, gradients, link, text), spacing (margin, padding), typography (fontSize, lineHeight)
 -	**Attributes:** content, level, placeholder, textAlign
 
 ## Home Link

lib/blocks.php

@@ -23,7 +23,6 @@ function gutenberg_reregister_core_block_types() {
 				'columns',
 				'comments',
 				'group',
-				'heading',
 				'html',
 				'list',
 				'list-item',
@@ -69,6 +68,7 @@ function gutenberg_reregister_core_block_types() {
 				'home-link.php'                    => 'core/home-link',
 				'image.php'                        => 'core/image',
 				'gallery.php'                      => 'core/gallery',
+				'heading.php'                      => 'core/heading',
 				'latest-comments.php'              => 'core/latest-comments',
 				'latest-posts.php'                 => 'core/latest-posts',
 				'loginout.php'                     => 'core/loginout',

packages/block-library/src/group/test/__snapshots__/edit.native.js.snap

@@ -3,7 +3,7 @@
 exports[`Group block inserts block and adds a Heading block as an inner block 1`] = `
 "<!-- wp:group {\\"layout\\":{\\"type\\":\\"constrained\\"}} -->
 <div class=\\"wp-block-group\\"><!-- wp:heading -->
-<h2></h2>
+<h2 class=\\"wp-block-heading\\"></h2>
 <!-- /wp:heading --></div>
 <!-- /wp:group -->"
 `;

packages/block-library/src/heading/block.json

@@ -29,7 +29,7 @@
 	"supports": {
 		"align": [ "wide", "full" ],
 		"anchor": true,
-		"className": false,
+		"className": true,
 		"color": {
 			"gradients": true,
 			"link": true,