A lone `-` should be treated as a regular positional argument

[SirNickolas]

It is a common idiom to accept a - parameter meaning stdin. E.g., cat - reads its whole stdin and sends it to stdout. However, argparse currently treats it as a named parameter, even though NamedArgument("") is not allowed—so it cannot be accessed in any way.

import argparse;

struct T {
    @NamedArgument bool c;
    @PositionalArgument(0) string fileName;
}

void main() {
    assert(CLI!T.parseArgs!((T t) { assert(t == T(true, "-")); })(["-", "-c"]) == 0);
}