fix: mutation validator and trigger cyclic import structure

[wschurman]

Why

Similar to #89 and #93, we need to do the same for validators and triggers in case those import entities themselves.

I tried rethinking this whole thing and just making the companion definition a function (static getCompanionDefinition(): () => EntityCompanionDefinition) but it made things less clean to define (had to remove top-level consts containing companion definitions to reap benefits) and made things less composable (because consts are no longer possible, can't be shared).

How

Make the params functions similar to other PR examples above.

Test Plan

Run all tests.

[codecov]

Codecov Report

Merging #118 (59e779f) into master (3ce961d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #118   +/-   ##
=======================================
  Coverage   95.08%   95.08%           
=======================================
  Files          67       67           
  Lines        1689     1689           
  Branches      202      202           
=======================================
  Hits         1606     1606           
  Misses         81       81           
  Partials        2        2           

Flag	Coverage Δ
integration	95.08% <100.00%> (ø)
unittest	95.08% <100.00%> (ø)

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

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

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