Block Library: Post Title: Title should be editable

[aduth]

Previously: #21467 (#21467 (comment)), #19572

Context:

#21467 made minimal enhancements to the Post Title block to show a live title of the post. Ideally, this title block should be editable and mirror to and from the corresponding post entity.

There is prior art in #19572 for this, where the title was implemented as RichText.

Task:

Implement an editable field for the Post Title block.

Implementation Specifics:

The title inherits many of the same behaviors of a heading block, and may warrant some refactoring to better support a feature parity.

See also:

• Feature parity between FSE Blocks and regular blocks #21087
• Common Block Functionality #15450

[mtias]

The title inherits many of the same behaviors of a heading block, and may warrant some refactoring to better support a feature parity.

Consider for editing, in most cases, those attributes would be locked (like the current post title ad hoc component we have in the main editor). When users are writing a post, they should be interacting with the title block, but they should not be able to change the heading level, etc, since they are not editing the template.

[ntsekouras]

Implemented here: #27240