Bugfixes

• Internal requested filename for import() with target: "electron-main" uses correct path separator on windows
  (This fixes a problem with filemappings in vscode)
• devtool: "source-map" and variants generate SourceMaps when output file is .mjs
• browser field as object is used when using target: "electron-renderer"
• Comments near export default are preserved
• Passing an array as externals value, now works correctly as documented