'main': Avoid triggering a zsh bug related to hashed commands.

This manifested in completion of the form «./foo<TAB>» where there happened to
be a program called 'foo' in $PATH.

Fixes #354.
Closes #355.

zsh-syntax-highlighting.zsh

@@ -383,5 +383,7 @@ add-zsh-hook preexec _zsh_highlight_preexec_hook 2>/dev/null || {
 # Load zsh/parameter module if available
 zmodload zsh/parameter 2>/dev/null || true
 
+autoload -U is-at-least
+
 # Initialize the array of active highlighters if needed.
 [[ $#ZSH_HIGHLIGHT_HIGHLIGHTERS -eq 0 ]] && ZSH_HIGHLIGHT_HIGHLIGHTERS=(main) || true