WIP: Add better type definitions

[mattphillips]

This is a bit of a POC to add type definitions for each diff function that works correctly.

I've not exhaustively tested this yet so don't know if it covers all cases but the general idea is as follows:

• diff: Merge the left hand side and right hand side types into optionals at all levels and convert arrays to dictionaries (recursing on the inferred array element type)
• added: Keep all new RHS keys and set all old LHS keys to never. All new new at any level should not be optional. Arrays can also have been inserted into so they should be kept in the types but made optional dictionaries.
• deleted: Keep all keys that are not present on the RHS keys but exist in the LHS as undefined. Arrays can also have been deleted from so they should be kept in the types but made optional dictionaries.
• updated: TODO

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Codecov Report

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

Project coverage is 100.00%. Comparing base (e37b759) to head (8a8138f). Report is 3 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #88 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 7 7 Lines 109 109 Branches 36 36 ========================================= Hits 109 109 ```

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