Upgrade the development toolchain to OCaml 5.5

[cknitt]

Summary

• use OCaml 5.5 for local development, the development container, and primary CI builds
• retain ocaml >= 5.0.0 as the package constraint and keep the dedicated OCaml 5.0 compatibility CI job
• upgrade ppxlib to 0.38, setup-ocaml to 3.7.2, and js_of_ocaml and wasm_of_ocaml to 6.4.1
• install a suitable Binaryen version for wasm_of_ocaml on CI and in the development container
• adapt the compiler pretty-printer to the OCaml 5.4 Format behavior change while keeping syntax-test output stable
• temporarily pin the pending bisect_ppx OCaml 5.5 compatibility fix for coverage builds

This is the fourth PR in the "Flow Forward" series. It is stacked on the OCamlformat 0.29 upgrade so the mechanical reformat and toolchain changes remain independently reviewable.

Related to #8567.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (codex/upgrade-ocamlformat-0.29@dfadefd). Learn more about missing BASE report.

Additional details and impacted files

@@                        Coverage Diff                        @@
##             codex/upgrade-ocamlformat-0.29    #8589   +/-   ##
=================================================================
  Coverage                                  ?   75.92%           
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  Files                                     ?      475           
  Lines                                     ?    63029           
  Branches                                  ?        0           
=================================================================
  Hits                                      ?    47854           
  Misses                                    ?    15175           
  Partials                                  ?        0           

Files with missing lines	Coverage Δ
compiler/ml/pprintast.ml	90.85% <100.00%> (ø)

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commit: aa9d92d

[cristianoc]

what are the formatting changes?

[cknitt]

what are the formatting changes?

Caused by upgrading ocamlformat.

[cristianoc]

what are the formatting changes?

Caused by upgrading ocamlformat.

That seems like a lot of churn on ocamlformat part. Strange. Isn't there a setting so we don't change everything?

[cristianoc]

Looks like there's no easy way out of the churn. If it turns out that there is, great. If not, OK.

[cknitt]

I checked this. OCamlformat 0.27 is constrained to OCaml <5.4, so we cannot keep it in the OCaml 5.5 toolchain. The newer formatter has changed some formatting rules, and there does not appear to be a compatibility setting for preserving the 0.27 output. I also tried restoring the previous exp-grouping default, but it produced the same changes.

To keep the OCaml 5.5 PR reviewable, I suggest moving the OCamlformat upgrade and mechanical reformatting into a separate PR in the stack.