fix: update setField to correct typing

[wschurman]

Why

4e40b2e incorrectly didn't update the set of fields that could be mutated for an entity that selects a subset of fields.

How

Update types.

Test Plan

yarn tsc

Use typeahead and see that only that entity's field subset are suggested.

[codecov]

Codecov Report

Merging #62 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   94.07%   94.07%           
=======================================
  Files          58       58           
  Lines        1384     1384           
  Branches      147      147           
=======================================
  Hits         1302     1302           
  Misses         81       81           
  Partials        1        1           

Flag	Coverage Δ
#integration	94.07% <100.00%> (ø)
#unittest	94.07% <100.00%> (ø)

Impacted Files	Coverage Δ
packages/entity/src/EntityMutator.ts	100.00% <100.00%> (ø)

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