zsh: support special parameters @ and * in parameter expansion with subscripts using braces #1299

# Echoes the last argument passed to it
fntest() { echo "${@[-1]}"; }; fntest foo bar baz
# baz

./shfmt --language-dialect zsh <<<'echo "${@[-1]}"'
# <standard input>:1:10: cannot index a special parameter name

./shfmt --language-dialect zsh <<<'echo "${*[-1]}"'
# <standard input>:1:10: cannot index a special parameter name

./shfmt --language-dialect zsh <<<'echo $@[-1]'
# echo $@[-1]
./shfmt --language-dialect zsh <<<'echo $*[-1]'
# echo $*[-1]