A bug of attachMacroArgsInArray

[Sean Lee]

\NewDocumentCommand{\xxx}{ e{_} }{#1}

$\xxx_1$

Here _ shouldn't be parsed as a macro, but rather as a part of an argument of \xxx,
but currently _1 is first gobbled as a macro and \xxx cannot find an argument.

Logic in attach-arguments.ts need to be changed.