Update WebPack's test regexps to match only js and jsx files

There is missing "end of string" in webpack's test regular expression. This
leads to testing also .json files (and any other file containing ".js" anywhere
in the string). See gatsbyjs/gatsby#10038 for one example of issue.

README.md

@@ -42,7 +42,7 @@ If you are for example using [Styled Components](https://www.styled-components.c
 module: {
   rules: [
     {
-      test: /\.jsx?/,
+      test: /\.jsx?$/,
       loader: 'stylelint-custom-processor-loader',
       exclude: /node_modules/,
     },
@@ -58,7 +58,7 @@ And you should now have linting your Styled Components css integrated with Webpa
 module: {
   rules: [
     {
-      test: /\.jsx?/,
+      test: /\.jsx?$/,
       use: [
         'babel-loader',
         'stylelint-custom-processor-loader',
@@ -77,7 +77,7 @@ Give the path to a non-default named Stylelint config. The recommended way to sp
 module: {
   rules: [
     {
-      test: /\.jsx?/,
+      test: /\.jsx?$/,
       use: [
         'babel-loader',
         {