docs: Make example code use CommonJS require()

Since ESM imports happen before the code is executed, mock-require
cannot be used (in theory) to mock ESM modules, unless they are
transpiled to CommonJS first. mock-require probably does not support
mocking native ES modules in Node.js

Eventually, someone may try to run the example using native ES modules,
and then complain that it doesn't work. To avoid confusion, let's
convert all examples to use CommonJS `require()`.

README.md

@@ -31,12 +31,12 @@ To test this code in Node.js, you can inject `xpath()` function into the global
 
 ```js
 // test/my-kolmafia-code.test.js
-import mock from 'mock-require';
-import {xpath} from 'kolmafia-stubs';
+const mock = require('mock-require');
+const {xpath} = require('kolmafia-stubs');
 
 mock('kolmafia', {xpath});
 
-import {myFunc} from './src/my-kolmafia-code';
+const {myFunc} = require('./src/my-kolmafia-code');
 ```
 
 ## Provided functions