build: DH-21044: Disable pre/post install scripts

[mofojed]

• Pre/post install scripts was something Sha1-hulud used to propagate
• Just disable them
• Updated scripts to ensure npm install then npm start/npm run build/npm run test all work correctly

[bmingles]

Left a couple of simplification suggestions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.22%. Comparing base (51c0a51) to head (b907175).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2586      +/-   ##
==========================================
- Coverage   45.48%   45.22%   -0.26%     
==========================================
  Files         771      771              
  Lines       43591    43591              
  Branches    11211    11027     -184     
==========================================
- Hits        19827    19716     -111     
- Misses      23748    23859     +111     
  Partials       16       16              

Flag	Coverage Δ
unit	45.22% <ø> (-0.26%)	⬇️

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[bmingles]

LGTM