Add description dedenting for help about run-fig

[clo4]

Until String.dedent is implemented, here's roughly what I'll do:

// If the string is empty, return the input.
// If the string doesn't start with a newline, return the input.
// Otherwise, trim the first line and the last line. Remove the same
// number of characters from the beginning of each line as the number
// of whitespace characters in front of the first line.
function dedent(str: string) {
  if (str[0] !== "\n") return str;
  const lines = str.split("\n").slice(1, -1);
  const margin = lines[0].length - lines[0].trimStart().length;
  return lines.map((line) => line.slice(margin)).join("\n");
}

(will benchmark implementations)