es5/client-legacy-bundle.js uses es6 syntax resulting in errors for older browsers

[rmi22186]

V9.11.0 (latest as of 10/10/2023)

The es5 directory is advertised as containing builds of fetch-mock which do not use any JS syntax not included in the [ES5 standard](https://es5.github.io/), i.e. excludes recent additions to the language. However, es5/client-legacy-bundle.js contains references to const and let, which are part of es6 syntax. While this will still run fine in modern browsers, when I run automated cross browser testing, there are syntax errors for older browsers due to them not recognizing const/let/other es6 syntax - One example is FF30. I know this is an old browser, but it was just a test to see if it worked or not. This bundle should be pure ES5 as advertised.

In my case I am using Browserstack to test on older browsers. When I do a global find replace for const and let, and change these to var and re-run, it points out that there is a for/of error which is also es6 (for/in being es5). It seems perhaps babel requires some configuration to get the transpiling process to run properly.