migrate to Accessors, move StaticArraysCore to ext

[longemen3000]

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

Additional context

I saw that SciMLBase depends both on Accessors and SetField. this PR migrates the uses of SetField to Accessors, and moves StaticArraysCore to an extension (in this way, StaticArraysCore is not loaded anymore)