fix: resolve ESLint module warning causing CI issues

Rename eslint.config.js to eslint.config.mjs to fix Node.js module
type warning that could cause GitHub Actions CI to treat warnings
as errors.

This resolves the MODULE_TYPELESS_PACKAGE_JSON warning without
breaking CommonJS compatibility for the main package.

✅ ESLint runs cleanly without module warnings
✅ All tests still pass
✅ VS Code extension builds successfully
✅ Ready for GitHub Actions CI

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Author: thegdsks