minor #8494 Changes manifest file extension from json to js. (Cifixie)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #8494).

Discussion
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Changes manifest file extension from json to js.

JSON as itself isn't valid Javascript causing an error when trying to parse as javascript: `manifest.json:2 Uncaught SyntaxError: Unexpected token :`  and it's only meant for Symfony asset versioning.

The bundler also creates `manifest.js` -file which creates `window.webpackJsonp` used in bundles. Without it, you might get an error: `app.315f91d41365513eb8ea.js:1 Uncaught ReferenceError: webpackJsonp is not defined`

Commits
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5d13c37 Changes manifest file extension from json to js.

Author: xabbuh

frontend/encore/shared-entry.rst

@@ -32,7 +32,7 @@ on your page before any other JavaScript file:
 .. code-block:: twig
 
     <!-- these two files now must be included in every page -->
-    <script src="{{ asset('build/manifest.json') }}"></script>
+    <script src="{{ asset('build/manifest.js') }}"></script>
     <script src="{{ asset('build/vendor.js') }}"></script>
 
     <!-- here you link to the specific JS files needed by the current page -->
@@ -42,7 +42,7 @@ on your page before any other JavaScript file:
     <link rel="stylesheet" href="{{ asset('build/vendor.css') }}" />
 
 The ``vendor.js`` file contains all the common code that has been extracted from
-the other files, so it's obvious that it must be included. The other file (``manifest.json``)
+the other files, so it's obvious that it must be included. The other file (``manifest.js``)
 is less obvious: it's needed so that Webpack knows how to load those shared modules.
 
 .. tip::