bug #732 feat: add ESLint 6 support (Kocal)

This PR was squashed before being merged into the master branch.

Discussion
----------

feat: add ESLint 6 support

Someone on Slack had an issue with Encore and ESLint 6:
![Capture d’écran de 2020-04-17 22-12-29](https://user-images.githubusercontent.com/2103975/79610285-8df35280-80f8-11ea-8175-5c9962478a69.png)

`CLIEngine#config` does not exists on ESLint 6 anymore, since [6ae21a4b](eslint/eslint@6ae21a4#diff-3daec82d5218d64d8f2fed4d21f8e6ccL472-L486).
Now we can use [`CLIEngine#getConfigForFile`](https://eslint.org/docs/developer-guide/nodejs-api#cliengine-getconfigforfile) to get the ESLint configuration.

Encore now supports both ESLint 5 and 6.

cc @a-menshchikov 😄

Commits
-------

aa8353f feat: add ESLint 6 support

Author: weaverryan

lib/loaders/eslint.js

@@ -12,6 +12,7 @@
 const WebpackConfig = require('../WebpackConfig'); //eslint-disable-line no-unused-vars
 const loaderFeatures = require('../features');
 const applyOptionsCallback = require('../utils/apply-options-callback');
+const logger = require('../logger');
 
 function isMissingConfigError(e) {
     if (!e.message || !e.message.includes('No ESLint configuration found')) {
@@ -35,7 +36,13 @@ module.exports = {
         });
 
         try {
-            engine.config.getConfigHierarchy(webpackConfig.runtimeConfig.context);
+            if (typeof engine.getConfigForFile === 'function') {
+                logger.debug('Checking ESLint 6+ configuration...');
+                engine.getConfigForFile('webpack.config.js');
+            } else {
+                logger.debug('Checking ESLint 5 configuration...');
+                engine.config.getConfigHierarchy(webpackConfig.runtimeConfig.context);
+            }
         } catch (e) {
             if (isMissingConfigError(e)) {
                 const chalk = require('chalk').default;

package.json

@@ -66,7 +66,7 @@
     "chai-fs": "^1.0.0",
     "chai-subset": "^1.6.0",
     "core-js": "^3.0.0",
-    "eslint": "^5.15.2",
+    "eslint": "^5.15.2 || ^6.0.0",
     "eslint-loader": "^2.1.2",
     "eslint-plugin-header": "^1.0.0",
     "eslint-plugin-import": "^2.8.0",