runtime(sh): allow "#" in special derefs

runtime(sh): allow "#" in special derefs

Code like ${!#} flags the "#" as shDerefWordError [1]; the "!prefix"
syntax region delegates to one of the shDerefSpecial handlers via
@shDerefList, but it misses the "#" case as valid for ${##} and ${!#}.

[1]: https://vi.stackexchange.com/q/48617/10604

Correct that. Indirection is only valid in Bash in Ksh, so rearrange the
"!" handling to be conditional.

closes: #20016

Helped-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>