zsh: support omitted parameter name in ${:-word} expansions #1280

Zsh allows the parameter name to be omitted in expansions:

This is commonly combined with the (%) prompt expansion flag, which enables
prompt escape expansion, see man zshmisc for details.

./shfmt -ln zsh <<<'echo ${:-word}'
# <standard input>:1:8: invalid parameter name

# Get name of script, sourced file, or function currently being executed (%N)
./shfmt -ln zsh <<<'echo ${(%):-%N}'
# <standard input>:1:11: invalid parameter name

# Get absolute path of the file containing the source code currently being executed (%x)
./shfmt -ln zsh <<<'echo ${${(%):-%x}:a}'
# <standard input>:1:13: invalid parameter name

# Powerlevel10k instant prompt (very common in the wild)
./shfmt -ln zsh <<<'source "${XDG_CACHE_HOME:-$HOME/.cache}/p10k-instant-prompt-${(%):-%n}.zsh"'
# <standard input>:1:66: invalid parameter name

zsh docs

${name-word}
${name:-word}

If name is set, or in the second form is non-null, then substitute its
value; otherwise substitute word. In the second form name may be
omitted, in which case word is always substituted.

Source: https://zsh.sourceforge.io/Doc/Release/Expansion.html#Parameter-Expansion

additional

zsh allows omitting the parameter name almost everywhere for its expansion
syntaxes, but the ${:-word} syntax seems the most useful to support

Related

a couple users mentioned the issue in #120

• syntax: zsh support #120 (comment)
• syntax: zsh support #120 (comment)
• syntax: zsh support #120 (comment)

EDIT1: typo