[StudentWeis]

Fixes #1405.

The problem

In Zsh mode, a double quote directly following $# in the short form
was treated as the start of a quoted parameter name. The parser then
consumed both the # and the " as a length prefix, failed to parse
a parameter name afterwards, and the expansion collapsed to a bare $:

$ printf 'if [ "$#" -lt 2 ]; then\n\techo "$#"\nfi\n' | shfmt -ln=zsh  # before
if [ "$" -lt 2 ]; then
	echo "$"
fi

This silently changes semantics, as $ expands to the shell PID in Zsh,
and zsh -n cannot detect it.

Root cause

paramNameStart accepted " as the start of a parameter name, which
is correct for the long form, where Zsh nested expansions in quotes
like ${#"${foo}"} exist (#1214). But in the short form the " is
never part of the expansion: real Zsh treats "$#" as the special
parameter, and $#"$foo" as $# followed by a quoted word:

% set -- x y; echo "$#"; echo $#"foo"
2
2foo

The same applied to the % and + short prefixes.

The fix

Pass whether the expansion is the short form into paramNameStart and
refuse a leading " in that case. The long form keeps accepting
quoted parameter names, so ${#"${foo}"} still parses as before.

Added parser regression tests for "$#" and $#"$foo".

[mvdan]

See: 019025c