core: move InsertPoint to Rewriter [NFC]

[superlopuh]

I'd like to add some helpers to Rewriter to leverage InsertPoint, and right now this would involve a circular reference. Between Rewriter and Builder, it feels like InsertPoint is closer in abstraction to Rewriter anyway.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.50%. Comparing base (b58fd69) to head (98e1a85).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2310      +/-   ##
==========================================
- Coverage   89.53%   89.50%   -0.03%     
==========================================
  Files         329      329              
  Lines       39429    39451      +22     
  Branches     5901     5906       +5     
==========================================
+ Hits        35304    35312       +8     
- Misses       3266     3277      +11     
- Partials      859      862       +3     

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[math-fehr]

I see where it's going and I like it