fix: add .js extensions to es build for Node ESM

[leno23]

Summary

Fixes #2528

Node ESM requires explicit .js extensions on relative imports. The es/ build output currently emits imports like from './util/assertString' without extensions, causing ERR_MODULE_NOT_FOUND when importing individual validators in Node.

This adds a post-build step (scripts/add-es-import-extensions.js) that runs after babel in build:es and appends .js to relative import/export paths in the generated es/ tree.

Also updates README examples to use the correct .js paths for both CommonJS and ESM.

Test plan

• npm run build:es completes successfully
• Node ESM import of ./es/lib/isEmail.js works
• npm test — 318 tests passing

[leno23]

Addressed the Node.js 8 CI failure in commit da8cfac. The build script no longer relies on fs.Dirent / withFileTypes, which Node 8 does not support; it now walks names from fs.readdirSync and checks directories with fs.statSync.

Local verification:

• node -c scripts/add-es-import-extensions.js

The shallow checkout did not include the generated es directory, so the full build remains covered by GitHub CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (da8cfac).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2720   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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