HTML API: Audit modified class_name whitespace handling

[jonsurrell]

When the HTML API modifies the class property via some of the class name oriented methods, the resulting whitespace may be inconsistent. For example, trailing whitespace is trimmed while leading whitespace is removed.

Audit this behavior to ensure it is clear and consistent.

​See this Slack conversation for some discussion.

The HTML API remove_class regarding white space opts to maintain "inter-class" whitespace.

​https://github.com/WordPress/wordpress-develop/blob/5effffccd005ca3dae6631ee73b1096b8f54a861/src/wp-includes/html-api/class-wp-html-tag-processor.php#L2223-L2230

This seems reasonable and justified, but normalization seems inconsistent. Leading space is maintained, but trailing space is not.

It seems more consistent to trim leading and trailing space, and actually maintain _only_ inter-class whitespace.

Trac ticket: https://core.trac.wordpress.org/ticket/61655

​This was discussed in Core Slack here.

@dmsnell How do you feel about moving ahead with this now?

[jonsurrell]

Duplicate of #61531.

Merged in ​[58740]
​https://github.com/wordpress/wordpress-develop/commit/2f1cf1f3db26e189d0782c3ddbd83a95a387f2b3