Remove unused compiler modules and helpers

[cknitt]

Follow-up to #8551, extending the compiler dead-code cleanup.

Summary

• Remove the unused js_fold and ast_exp modules.
• Remove unused helpers and interface declarations across the compiler’s core, frontend, type-checking, syntax, and utility layers.
• Remove dependency error variants that could never be constructed.
• Remove Typedtree fields that were populated but never read.
• Update the error-variant catalog accordingly.

Overall, this removes 2,241 lines across 126 files without changing compiler behavior.

Testing

• make test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.54%. Comparing base (1beb54d) to head (488c3b6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8555      +/-   ##
==========================================
+ Coverage   74.88%   75.54%   +0.65%     
==========================================
  Files         476      476              
  Lines       63269    62710     -559     
==========================================
- Hits        47382    47374       -8     
+ Misses      15887    15336     -551     

Files with missing lines	Coverage Δ
compiler/common/js_config.ml	75.00% <ø> (+8.33%)	⬆️
compiler/core/js_cmj_format.ml	89.79% <ø> (+6.77%)	⬆️
compiler/core/js_cmj_load.ml	80.00% <ø> (ø)
compiler/core/js_dump.ml	87.30% <ø> (ø)
compiler/core/js_exp_make.ml	64.35% <ø> (+1.05%)	⬆️
compiler/core/js_of_lam_block.ml	100.00% <ø> (+11.11%)	⬆️
compiler/core/js_of_lam_option.ml	100.00% <ø> (+3.22%)	⬆️
compiler/core/js_of_lam_string.ml	100.00% <ø> (+50.00%)	⬆️
compiler/core/js_packages_info.ml	73.91% <ø> (+18.17%)	⬆️
compiler/core/lam.ml	70.66% <ø> (+13.47%)	⬆️
... and 53 more

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[pkg-pr-new]

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@rescript/win32-x64

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commit: 488c3b6

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8555

[cristianoc]

Great lots of unused stuff found.