Aliases must be resolved before we check command argument types

[Infernio]

Otherwise you could get into situations where e.g. a Word-typed argument has to reject all aliases since one of them could later expand to more than a word, or a Text-typed argument has an alias that then expands into commands etc.

The underlying reason for this is because every alias has the type Content:

\def_alias[
    key: Word;
    val: Content;
] -> None

The val is the newly defined alias' value, which has type Content. So we have to resolve the alias first to verify the actual type matches.