parseStringGroup(e, r) {
              const l = this.gullet.scanArgument(r);
              if (l == null)
                return null;
              let c = "", m;
              for (; (m = this.fetch()).text !== "EOF"; )
                c += m.text, this.consume();
              return this.consume(), l.text = c, l;
            }
Parses an URL, checking escaped letters and allowed protocols,
and setting the catcode of % as an active character (as in \hyperref).