zsh older than workers/28418 (zsh revision 23bdfc7fd2)
doesn't support the «${name:offset:length}» syntax. Therefore, use the older
«$name[start,end]» syntax instead.
Followup to 3cb58fd7d7b9e42067ffcf24892114dc2c9b26e4, "Optimization: -1 is
extra computation of string length internally".
Fixeszsh-users/zsh-syntax-highlighting#279.