fix: edge deletion behavior cyclic import structure again

[wschurman]

Why

Apparently #89 wasn't sufficient for all cases. With the way entities are encouraged to be written, it seems that even with one side of this cycle being defined as a function isn't sufficient to squash all import cycles. I think this is a fairly unique case though as it is literally defining a cycle so I still don't think that having the whole companion or configuration be a function is necessary (don't foresee any other future features needing to specify other entities in configurations).

How

Make the other direction edge specification a function as well.

Test Plan

This one is harder to repro using a simple test case. To test this, I yarn link'ed the entity package into Expo code where we're seeing the behavior and ensured it fixed it.

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   94.70%   94.71%           
=======================================
  Files          62       62           
  Lines        1550     1551    +1     
  Branches      166      166           
=======================================
+ Hits         1468     1469    +1     
  Misses         80       80           
  Partials        2        2           

Flag	Coverage Δ
integration	94.71% <100.00%> (+<0.01%)	⬆️
unittest	94.71% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/entity/src/EntityFields.ts	100.00% <ø> (ø)
.../src/__integration-tests__/entities/ChildEntity.ts	100.00% <100.00%> (ø)
...src/__integration-tests__/entities/ParentEntity.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityConfiguration.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityMutator.ts	98.00% <100.00%> (ø)

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[ide]

I'm a little wary of leaving the name as is instead of renaming it to getAssociatedEntityClass since classes are functions in JS so it's easy to get the two mixed up but this looks fine otherwise for the cyclic entity case.