Expose independent variable as a system property

[hersle]

This is a suggestion to expose the independent variable as a system property, on equal footing with variables and parameters.

I think this is more consistent than not doing it. If a user is handed a system sys from somewhere outside their control (like a factory function that uses t internally), it also makes it possible for them to easily obtain sol[sys.t] without any workaround.

What do you think about this?

Checklist

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