Support highlighting of a single and multiple cells. Similar to the current boxed expression editor:

Notice: only editable cells must be selectable, to avoid scenarios like following image shows. The main goal of multiple selection, in the new boxed expression editor, is supporting copy/paste integration with other tools (like Google Sheets). We must not highlight context keys, as they are complex cells (they have a name and a type), the new editor won't support copy/paste on them, for now.