Update TinyMCE to no longer strip <picture>, <source> tags

Update TinyMCE to no longer strip <picture>, <source> tags

When developing email templates that work with both light and dark mode, using <picture> and <source> tags with a fallback <img> inside it is a clean way to set both light and dark mode images without having to assign the image a conditional media query show/hide via CSS. However, the TinyMCE editor today will strip this markup rather than allow it to be in editable areas. Can this please be updated to no longer have this behavior?