Fix module resolving for chai

Eslint was not able to load chai with webpack import loader due error:
Package path . is not exported from package /node_modules/chai (see exports field in chai/package.json)
Pointing directly to node_modules/chai/index.js bypasses checking exports property in package.json
[MAILPOET-3214]

webpack.config.js

@@ -55,6 +55,7 @@ const baseConfig = {
       'spectrum$': 'spectrum-colorpicker/spectrum.js',
       'wp-js-hooks': path.resolve(__dirname, 'assets/js/src/hooks.js'),
       'blob$': 'blob-tmp/Blob.js',
+      'chai': 'chai/index.js',
       'papaparse': 'papaparse/papaparse.min.js',
       'html2canvas': 'html2canvas/dist/html2canvas.js',
       'asyncqueue': 'vendor/jquery.asyncqueue.js',