code: function insertHandler(string, selectionRange, snippet) {
    const start = selectionRange[0];
    const end = selectionRange[1];
    const value = string.substring(0, start) + snippet + string.substring(end, string.length);
  
    return { value, range: [start, start + snippet.length] };
  }
docs: Handles inserting a snippet at the end of a selection
  @param   {String} string         the entire string to use
  @param   {Number[]}  selectionRange the starting and ending positions of the selection
  @param   {String} snippet        the snippet to insert
  @returns {Object} the new string, the updated selectionRange