fix(vanilla): wrap bundles in IIFE to avoid duplicate global __webpack_exports__ #55

Summary

Vanilla JS bundles from getVanillaConfig() set output.iife: false, so each file emits a top-level let __webpack_exports__ in global scope. When two or more such bundles load on the same WordPress admin page, Safari throws:

SyntaxError: Can't create duplicate variable: '__webpack_exports__'

Reactified builds (getReactifiedConfig()) are unaffected — they are wrapped in an IIFE.

Steps to reproduce

1. Build a Polylang consumer (e.g. Polylang Pro) with npm run dev.
2. Open an admin screen that enqueues multiple vanilla scripts (e.g. posts list → pll_admin + pll_post, or media library → pll_admin + pll_media).
3. Check the browser console.

Suggested fix

Set iife: true in transformJsEntry() (and in createBundledJsConfig() if present):

output: {
  // ...
  iife: true,
},

Vanilla scripts are standalone WordPress enqueues with no global library export, so scoping webpack internals should not change runtime behaviour.

Acceptance Tests

• npm test in polylang-build-scripts
• Polylang Pro builds and loads without console errors on admin screens with 2+ vanilla scripts
• Smoke-test: languages settings, widgets, classic editor, bulk translate
• Reactified bundles (editors/site.js, block-editor.js) unchanged

Environment

• @wpsyntex/polylang-build-scripts 2.4.0, webpack 5.x
• Consumer: Polylang Pro 3.9-beta2
• Reproducible with SCRIPT_DEBUG on or off (.js and .min.js)