Special 'span' used as a marker to know there's a difference between the indexes in the Rust code and the ones in the UI components.

This is done because in the Dom, just using a <li> tag will create a new line break in a list, but for Android we have to add an extra \n line break character for list items to be rendered properly. To fix this mismatch, we use this ExtraCharacterSpan to understand when a line break character should be left as is, because it exists in the ComposerModel or if we should handle it in a special way because it's not present in the HTML output.