Support Moshi wildcards via dispatch

[visr]

Fixes #945
Reported upstream as Roger-luo/Moshi.jl#43.

This should restore the old behavior. One possible exception is is_discrete_time_domain, which was defined as not being continuous. So that previously returned true for non-clocks, which seems undesirable to me. Though I'd be happy to preserve that behavior if it is expected anywhere.

[visr]

Ha I missed #944, which already fixes #945. I just rebased this on top of that because it's still worth fixing this behavior on master, which is unrelated to the Moshi change:

julia> is_discrete_time_domain(:banana)
true

And it is probably a bit better to use dispatch here instead of isa.

[AayushSabharwal]

Ha I missed #944, which already fixes #945.

Haha yeah this PR had me confused for a minute, "Wait didn't this already merge? And wasn't the author someone else?"

[AayushSabharwal]

While I agree it does make sense that is_discrete_time_domain return false for non-time-domain arguments, it has been the way it is for a long time. I'm concerned this may break things that somehow rely on the weird behavior. MTK CI isn't running right now - SciML/ModelingToolkit.jl#3382 will fix that and once it passes I'll approve this.