chore(babel): modernize config and clean webpack

babel.config.json

@@ -0,0 +1,5 @@
+{
+    "babelrcRoots": [
+        "."
+    ]
+}

webpack.config.cjs

@@ -1,35 +1,38 @@
-const fs = require('fs');
 const path = require('path');
 const ESLintPlugin = require('eslint-webpack-plugin');
 
 const mode = process.env.NODE_ENV;
 const noInline = process.env.noInline;
 const debugBuild = mode === 'development';
 
-/*
-   configuring babel:
-   - when babel runs alone (for `test-unit` for instance), we let him deal with
-   ES6 modules, because node doesn't support them yet (planned for v10 lts).
-   - however, webpack also has ES6 module support and these 2 don't play well
-   together. When running webpack (either `build` or `start` script), we prefer
-   to rely on webpack loaders (much more powerful and gives more possibilities),
-   so let's disable modules for babel here.
-   - we also dynamise the value of __DEBUG__ according to the env var
-*/
-// Note that we don't support .babelrc in parent folders
-const babelrc = fs.readFileSync(path.resolve(__dirname, '.babelrc'));
-const babelConf = JSON.parse(babelrc);
-
-babelConf.babelrc = false; // disabel babelrc reading, as we've just done it
-const replacementPluginConf = babelConf.plugins.find(plugin => Array.isArray(plugin) && plugin[0] === 'minify-replace');
-replacementPluginConf[1].replacements.find(decl => decl.identifierName === '__DEBUG__').replacement.value = debugBuild;
+// For each file, this config is merged with its local babel configuration.
+// See https://babeljs.io/docs/configuration#how-babel-merges-config-items
+const babelConf = {
+    rootMode: 'upward',
+    plugins: [
+        ['minify-replace', {
+            replacements: [{
+                identifierName: '__DEBUG__',
+                replacement: {
+                    type: 'booleanLiteral',
+                    value: debugBuild,
+                },
+            }],
+        }],
+    ],
+};
 
 const include = [
     path.resolve(__dirname, 'src'),
     path.resolve(__dirname, 'test'),
     path.resolve(__dirname, 'utils'),
 ];
 
+const exclude = [
+    path.resolve(__dirname, '.git'),
+    path.resolve(__dirname, 'node_modules'),
+];
+
 module.exports = () => {
     const babelLoaderOptions = [];
     if (!noInline) {
@@ -72,6 +75,7 @@ module.exports = () => {
             rules: [
                 {
                     test: /\.js$/,
+                    exclude,
                     include,
                     use: babelLoaderOptions,
                 },
@@ -89,7 +93,7 @@ module.exports = () => {
             static: {
                 directory: path.resolve(__dirname, './'),
                 watch: {
-                    ignored: [path.resolve(__dirname, '.git'), path.resolve(__dirname, 'node_modules')],
+                    ignored: exclude,
                 },
             },
             client: {