Generalize symbolic indexing of ODE/DDE to multivariate variables

[hersle]

This is one possible fix to https://github.com/SciML/ModelingToolkit.jl/issues/3737. It generalizes a line that assumed DDE variables are like $x(t-\tau)$ to permit multivariate $x(t-\tau, a_2, a_3, \ldots)$. Indexing ODEs with multivariate variables works "for free" when the DDE line no longer errors.

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API