Edit the formulas of the MathJax macro in place in the WYSIWYG editor

The WYSIWYG editor shows the formulas of the MathJax macro as raw LaTeX in a read-only widget. Changing a formula needs opening the Edit Macro dialog, where it is a plain text area, with no formula editing aid and no preview.

The fix is done in MathJaxMacro.xml: edit the formula in place, in a code area for its LaTeX source, ideally with a preview of the typeset formula next to it. An empty formula must keep that code area visible, so that one click puts the cursor in it, also right after the insertion. MATHJAX-39 covers showing the formula typeset in the editor, which this editing builds on.

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.