feat: support `setindex!` for nonlinear integrators

[AayushSabharwal]

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

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.73%. Comparing base (a74c321) to head (5eba972).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #447 +/- ## ========================================== - Coverage 86.45% 85.73% -0.73% ========================================== Files 47 47 Lines 2872 2874 +2 ========================================== - Hits 2483 2464 -19 - Misses 389 410 +21 ```

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