refactor: Remove children field from ASTNode

[samestep]

Description

Resolves #841. The only place where a children function was actually necessary is synthesis/Search, so I just wrote one there using a very simple switch statement.

Implementation strategy and design decisions

I removed the traverseTree checks in our parser tests, because all they seem to do is check each node's nodeType, but that is already guaranteed via how each of our *.ne files defines a nodeData constant which sets nodeType.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new ESLint warnings
• I have reviewed any generated changes to the diagrams/ folder

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[codecov]

Codecov Report

Merging #956 (ee905ac) into main (199e9c6) will decrease coverage by 0.06%.
The diff coverage is 55.26%.

@@            Coverage Diff             @@
##             main     #956      +/-   ##
==========================================
- Coverage   62.82%   62.75%   -0.07%     
==========================================
  Files          62       62              
  Lines        7857     7889      +32     
  Branches     1821     1842      +21     
==========================================
+ Hits         4936     4951      +15     
- Misses       2797     2814      +17     
  Partials      124      124              

Impacted Files	Coverage Δ
packages/core/src/analysis/SubstanceAnalysis.ts	75.82% <ø> (ø)
packages/core/src/compiler/Domain.ts	86.22% <ø> (ø)
packages/core/src/compiler/Substance.ts	92.36% <ø> (ø)
packages/core/src/engine/EngineUtils.ts	64.74% <ø> (ø)
packages/core/src/engine/Evaluator.ts	32.55% <ø> (ø)
packages/core/src/parser/ParserUtil.ts	97.01% <ø> (ø)
packages/core/src/synthesis/Synthesizer.ts	28.13% <ø> (ø)
packages/core/src/types/ast.ts	100.00% <ø> (ø)
packages/core/src/synthesis/Search.ts	77.29% <54.05%> (-4.86%)	⬇️
packages/core/src/compiler/Style.ts	78.83% <100.00%> (-0.02%)	⬇️

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[wodeni]

Looks great! This is much cleaner and easier to maintain.