zsh older than workers/28418 (zsh revision 23bdfc7fd2a012d5205ed22d18eb39e41c8fbc95)
doesn't support the «${name:offset:length}» syntax. Therefore, use the older
«$name[start,end]» syntax instead.
Followup to 3cb58fd7d7, "Optimization: -1 is
extra computation of string length internally".
Fixes zsh-users/zsh-syntax-highlighting#279.
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES keys) each highlighter defines.
Refer to the documentation on highlighters for further information.