'main': Restore support for zsh-4.3.10 and older.

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.

highlighters/main/main-highlighter.zsh

@@ -235,13 +235,11 @@ _zsh_highlight_main_highlighter()
       # indistinguishable from 'echo foo echo bar' (one command with three
       # words for arguments).
       local needle=$'[;\n]'
-      # Len-start_pos drops one character, but it should do it, as start_pos
-      # starts from next, not from "start_pos", character
-      integer offset=${${buf: start_pos: len-start_pos}[(i)$needle]}
+      integer offset=${${buf[start_pos+1,len]}[(i)$needle]}
       (( start_pos += offset - 1 ))
       (( end_pos = start_pos + $#arg ))
     else
-      ((start_pos+=(len-start_pos)-${#${${buf: start_pos: len-start_pos}##([[:space:]]|\\[[:space:]])#}}))
+      ((start_pos+=(len-start_pos)-${#${${buf[start_pos+1,len]}##([[:space:]]|\\[[:space:]])#}}))
       ((end_pos=$start_pos+${#arg}))
     fi