Make library work with more browsers by polyfilling replaceAll()

[Jaifroid]

In this file https://github.com/kiwix/libkiwix/blob/main/static/skin/viewer.js#L76, a replaceAll() method is used that is incompatible with a number of older browsers. We at Kiwix JS have found several cases where the fancy library cannot be used simply because of this method (e.g., any Chrome before 85, see https://caniuse.com/?search=replaceAll), even though the browser would otherwise support the library feature.

We tried to polyfill it ourselves, but due to CORS its' not possible for us to tamper with this script.

For more info see kiwix/kiwix-js#1174 and kiwix/kiwix-js#1190. And for a very small polyfill that could simply be added to the viewer.js without needing to download a package, see https://github.com/kiwix/kiwix-js/pull/1190/files.