Show and edit the text of the comment macro in place in the WYSIWYG editor

The comment macro renders nothing, so the WYSIWYG editor shows only the generic "macro:comment" placeholder: reading or changing a comment needs opening the Edit Macro dialog, where it is a plain text area. For example, {{comment}}Remember to update this table every release.{{/comment}} shows only "macro:comment" in the editor.

The fix is done in CommentMacro.java: when the macro is rendered for the WYSIWYG editor, output the text of the comment in an area that is edited in place and styled as a comment, while view mode keeps rendering nothing. An empty comment must keep that area, compact but reachable by one click, rather than the "macro:comment" placeholder, which one click selects but never puts the cursor in. XWIKI-25523 is related: it standardizes how a macro knows that it is rendered for the WYSIWYG editor.

KOs covered

Impact found on macro fluidity

These 2 KOs come from the WYSIWYG usability audit of the macros, under the criteria E6 inplace and E5 empty of its usability criteria, which ask that the content is edited in place, with an editing experience suited to its data, not in a text area of the Edit Macro dialog, and that an empty content stays visible and editable, with no area that cannot receive the cursor. The page of the macro, linked in the table, has the full notes and the steps to reproduce. It is also needed by XWIKI-25563, so that the Quick Action can insert the macro without the Edit Macro dialog.