Prune unused vendored Flow parser modules

[cknitt]

Summary

• remove unused ESTree, JSDoc, location, and token translation modules from the vendored Flow parser
• remove unused collection helpers
• list retained vendored modules explicitly in Dune
• document why the remaining parser modules are required

This removes approximately 4,450 lines, or 12% of the vendored OCaml sources.

Further mechanical pruning is not feasible because ReScript uses the parser for both complete JavaScript programs and individual expressions in %raw. The Flow parser connects expression parsing transitively to statements, declarations, types, JSX, patterns, comments, and its shared AST.

Removing substantially more would therefore require disabling accepted syntax or restructuring the parser, rather than simply deleting unused modules. That would need a clearly defined %raw JavaScript syntax contract and broader parser-specific test coverage.

This is the fifth PR in the “Flow Forward” series, stacked on the OCaml 5.5 toolchain upgrade.

Related to #8567.

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commit: 13ed511

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.92%. Comparing base (9be2ff5) to head (13ed511).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8593   +/-   ##
=======================================
  Coverage   75.92%   75.92%           
=======================================
  Files         475      475           
  Lines       63015    63029   +14     
=======================================
+ Hits        47842    47854   +12     
- Misses      15173    15175    +2     

see 25 files with indirect coverage changes

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[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8593