minor #10042 [Encore] Fix vue-loader installation (dunglas)

This PR was merged into the 3.4 branch.

Discussion
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[Encore] Fix vue-loader installation

Encore isn't compatible yet with `vue-loader` 15 (symfony/webpack-encore#311). It leads to a broken install with a non-obvious error message.

In the meantime, we should hint users to install v14.

Commits
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c48e4b4 [Encore] Fix vue-loader installation

Author: javiereguiluz

frontend/encore/vuejs.rst

@@ -5,7 +5,7 @@ Want to use `Vue.js`_? No problem! First, install Vue and some dependencies:
 
 .. code-block:: terminal
 
-    $ yarn add --dev vue vue-loader vue-template-compiler
+    $ yarn add --dev vue vue-loader@^14 vue-template-compiler
 
 Then, activate the ``vue-loader`` in ``webpack.config.js``: