Export more functions relating to system initialization

[hersle]

I use these quite often to inspect my systems, so I suggest to export them.
Intuitively, I expect defaults(sys), initialization_equations(sys), ... to work because unknowns(sys), equations(sys), ... works.

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