fix: add fullySpecified: false to webpack config for v2 addon index.js resolution

[lifeart]

Adds fullySpecified: false to webpack's module rules to support v2 addons that use "type": "module" in their package.json with directory imports.

Problem

When a v2 addon uses "type": "module" in package.json and has internal imports without explicit /index.js paths, webpack 5's strict ESM resolution fails to resolve them.

Error Case (before fix)

  Addon structure:                                                                                                                                                                             
  my-addon/                                                                                                                                                                                    
  ├── package.json          # { "type": "module", "exports": { ".": "./dist/index.js", "./*": "./dist/*" } }                                                                                   
  └── dist/                                                                                                                                                                                    
      ├── index.js          # import { foo } from './components/my-component'                                                                                                                  
      └── components/                                                                                                                                                                          
          └── my-component/                                                                                                                                                                    
              └── index.js  # export function foo() { ... }                                                                                                                                    

Error message:

  Module not found: Error: Can't resolve './components/my-component'                                                                                                                           

BREAKING CHANGE: The request './components/my-component' failed to resolve only because
it was resolved as fully specified (probably because the origin is strict EcmaScript Module,
e.g. a module with javascript mimetype, a '.mjs' file, or a '.js' file where the
package.json contains '"type": "module"').
The extension in the request is mandatory for it to be fully specified.
Add the extension to the request.

Success Case (after fix)

The same addon structure now works correctly:

• import { foo } from './components/my-component' resolves to ./components/my-component/index.js
• Directory imports within ESM packages work without requiring explicit /index.js suffix

Solution

Add a webpack module rule that disables strict ESM resolution for .js files:

{                                                                                                                                                                                            
  test: /\.js$/,                                                                                                                                                                             
  resolve: {                                                                                                                                                                                 
    fullySpecified: false,                                                                                                                                                                   
  },                                                                                                                                                                                         
},                                                                                                                                                                                           

• https://webpack.js.org/configuration/module/#resolvefullyspecified

[lifeart]

Closing this PR because it's seems cover same issue we had to fix but from another side:

The fullySpecified: false fix:

• Allows webpack to resolve directory imports like ./components/my-component -> ./components/my-component/index.js
• Fixes webpack's strict ESM resolution for type: "module" packages

Our current fix (#683):

• Ensures the AMD dependency lookup uses resolvedSpecifier (what webpack actually resolved to) instead of requestedSpecifier (what was written in the import)

They're complementary:

Import: 'addon-with-index/components/my-component'
->
fullySpecified: false
->
Webpack resolves to: 'addon-with-index/components/my-component/index.js'
->
Our fix ensures EAI_DISCOVERED_EXTERNALS uses this resolved path
->
AMD dependency lookup succeeds

Without both fixes:

1. Without fullySpecified: false -> webpack can't resolve directory imports at all
2. Without our fix -> even if webpack resolves correctly, AMD lookup uses wrong specifier and can't find dependencies