Add loadByIDNullableAsync loader method

[wschurman]

Why

One common use case we're noticing is wanting to check if an ID corresponds to one table or another (only useful for UUID primary key tables).

Another use case is determining whether an ID that should exist truly does (loading permalink page). While this can be handled cleanly with a good error boundary, it still makes sense to have this API available when preferred.

Closes #12.

How

Add method, enforcing method, and tests. Also add test to ensure consistent API between loader and enforcing loader.

Test Plan

Run tests.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   88.48%   88.51%   +0.03%     
==========================================
  Files          63       63              
  Lines        1138     1141       +3     
  Branches       97       98       +1     
==========================================
+ Hits         1007     1010       +3     
  Misses         62       62              
  Partials       69       69              

Flag	Coverage Δ
#integration	88.51% <100.00%> (+0.03%)	⬆️
#unittest	88.51% <100.00%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
packages/entity/src/EnforcingEntityLoader.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityLoader.ts	85.41% <100.00%> (+0.31%)	⬆️
packages/entity/src/entityUtils.ts	86.48% <0.00%> (ø)

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[quinlanj]

thanks!