errors: add ERR_AMBIGUOUS_MODULE_SYNTAX message

[Divya9877]

Summary

Adds an error message for ERR_AMBIGUOUS_MODULE_SYNTAX, which is used when a file mixes CommonJS and ES Module syntax.

Changes

• Registered ERR_AMBIGUOUS_MODULE_SYNTAX in lib/internal/errors.js with a clear explanatory message.

Why

This improves developer experience by explaining that the file mixes CommonJS (require, module.exports) with ES Module features (import, export, top-level await) and suggests using a single module system consistently.

[Divya9877]

Hi maintainers 👋,

This PR adds a test case for the new ERR_AMBIGUOUS_MODULE_SYNTAX error.

🔹 What I added

A new test at:
test/es-module/test-ambiguous-module-syntax.js

A new fixture at:
test/fixtures/es-modules/ambiguous-mixed-syntax.js

🔹 What the test does

Uses the mixed-syntax fixture to simulate ambiguous ESM/CJS usage.

Runs the module through Node.

Asserts that Node throws the expected error code:
ERR_AMBIGUOUS_MODULE_SYNTAX.

Ensures the new error message is correctly triggered and validated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (c6316f9) to head (4b35ee1).
⚠️ Report is 562 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60968      +/-   ##
==========================================
+ Coverage   88.45%   88.52%   +0.06%     
==========================================
  Files         703      703              
  Lines      207546   208437     +891     
  Branches    40011    40200     +189     
==========================================
+ Hits       183591   184524     +933     
+ Misses      15949    15903      -46     
- Partials     8006     8010       +4     

Files with missing lines	Coverage Δ
lib/internal/errors.js	97.52% <100.00%> (+<0.01%)	⬆️

... and 268 files with indirect coverage changes

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[aduh95]

Tests are not passing, and the linked issue was closed as resolved. Closing for now, thanks for the PR.