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.