Post Comments Form: Make heading customizable

[ockham]

What problem does this address?

Required to fix #43203. Quoting the most relevant part:

Pulling out of this trac issue on adding an "accessibility ready" tag for the Twenty Twenty-Two Theme. From @carolinan who did the audit:

On single posts or pages, when comments are enabled, the site title is an H1, the post title is an H1, but the next title is the "Leave a Reply" title and it is an H3, so the H2 level is skipped.
If there are comments, the "One response to “title” is present, but it is also an H3:
H1 Site Title
H1 Sample Page
H3 One response to “Sample Page”
H3 Leave a Reply

While TT2 uses the (legacy) Post Comments block, a similar issue applies to block themes that use the (newer) Comments block, which in turn uses the Comments Title and Post Comments Form block: While the Comments Title block ("One response to...") allows customization of the heading level (and defaults to H2), the Post Comments Form block is a thin wrapper around the comment_form() function (docs). While that function supports customization of the heading, it is not currently exposed via block controls, and defaults to H3. This means that there still is an a11y issue for posts that don't have any comments posted (yet):

H1 Sample Page
H3 Leave a reply

What is your proposed solution?

As discussed in WordPress/wordpress-develop#3136 (comment): Make the Post Comments Form block heading level customizable.

The required code change for the frontend is pretty simple:

comment_form(
	array(
		'title_reply_before' => '<h2 id="reply-title" class="comment-reply-title">',
		'title_reply_after'  => '</h2>'
	)
);

Note that in the long run, we plan to turn the "monolithic" Post Comments Form block into a "composable" block with child blocks such as Comment Author Field, Comment Content field, etc (#38107) -- much like we've already done when introducing the Comments block as a replacement for the Post Comments one. However, there's no ETA for this, and we might want to offer this basic customization of the heading level already.

IMO, the big question is if it's weird to have "only" one (somewhat random?) customization option on the Post Comments Form block -- and if it's weird to have a heading level control on a block that's more than just a heading. Curious to hear our designers' thoughts 😄

[richtabor]

Related #40320