WP_HTML_Tag_Processor: Rename attribute_updates to lexical_updates

[ockham]

What?

Rename:

• The $attribute_updates member to $lexical_updates
• apply_attribute_updates() to apply_lexical_updated()
• class_name_updates_to_attributes_updates() to class_name_updates_to_lexical_updates

And update PHPDoc and comments accordingly.

All of these are private, so we're not breaking any APIs.

Why?

To enable other kinds of updates via the same mechanism, e.g. for a prospective set_content_inside_balanced_tags(). See #47036 and #46680 (comment) for more context.

How?

It's a simple rename 😬

Testing Instructions

See unit tests.

[github-actions]

Flaky tests detected in b24d6c7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/3930915630
📝 Reported issues:

• [Flaky Test] should always expand single line selection #39787 in specs/editor/various/multi-block-selection.test.js
• [Flaky Test] should forward delete across blocks #44353 in specs/editor/various/multi-block-selection.test.js
• [Flaky Test] should insert media, convert to blocks, and undo in one step #36123 in specs/editor/blocks/classic.test.js
• [Flaky Test] should multi-select in the ListView component with shift + up and down keys #44354 in specs/editor/various/multi-block-selection.test.js

[dmsnell]

If it helps then I think we can merge this now as a rename and consider the deeper implications as we go. At least, this is an easy change to review. Thanks 👍