Debugging: generate source map (#65)

Source maps are useful to debug minified code. It allows developer to
request "the original source code" when using Developer Tools on Firefox
(or Chrome) and be able to debug it in production. Since our script is
open-source and its repository is public, I don't think there is any
security concern we should care about here (at this point, at least).

However, regular users will only get the minified version of it.

Note this option adds an extra ~1s to `npm run build` and `npm run dev`,
but I think that's not a problem for our use case.



![Screenshot_2023-05-01_10-31-49](https://user-images.githubusercontent.com/244656/235497668-92e47971-9c76-425e-9dd7-6ccc8d8e9032.png)


Reference: https://webpack.js.org/configuration/devtool/
Reference:
https://webpack.js.org/plugins/terser-webpack-plugin/#terseroptions

Author: humitos

dist/readthedocs-client.js

@@ -41,6 +41,9 @@ module.exports = (env, argv) => {
         new TerserPlugin({
           // Avoids creating a `.LICENSE.txt` file
           extractComments: false,
+          terserOptions: {
+            sourceMap: true,
+          },
         }),
       ],
     },
@@ -76,6 +79,7 @@ module.exports = (env, argv) => {
     ],
 
     // Development options
+    devtool: "source-map",
     watchOptions: {
       aggregateTimeout: 1000,
       ignored: ["/node_modules/", "**/node_modules/"],