The ZSH manual describes `region_highlight` as being an array in
https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html#Character-Highlighting,
therefore the previous strategy of removing as many characters as the
last suggestion is *not* the way to do it, explaining why it broke on
recent ZSH versions.
Replace this logic with a simple last-element delete. Keeps the
`_ZSH_AUTOSUGGEST_LAST_HIGHLIGHT` variable intact since there's no
downside in tracking its content, as it still used as a marker for
whether a suggestion highlight was applied.
This fixes a small issue in src/widgets.zsh which makes it so if you
alias [ to g[ (as is done in prezto if the gnu-utility module is loaded)
autosuggestions would fail.
The documentation for GNU test mentions that -o and -a should be avoided
if possible because it's not very clear. Also, with zsh and [[ -o
actually tests if an option is set, which makes this option even more
confusing.
