Validate argument input

Currently, there is no documented way to validate the argument input using usage. It is possible to do this with a simple shell script passed as `parse`, but this is kludgey and prone to error. This is a substantial limitation for `mise` relative to `just` which accomplishes this with [regex](https://just.systems/man/en/recipe-parameters.html?highlight=pattern#recipe-parameters).

Perhaps there is some undocumented way to accomplish this?