(docs): use window-resizeto polyfill to simplify code

[agilgur5]

• instead of polyfilling it oneself or introducing potentially confusing code

• and the /polyfill import can also be recommended for Jest environments for even greater simplicity

See window-resizeto's docs for more info and different use-cases (including window-resizeto/polyfill). It was made to solve this issue and re-use test code between projects so one doesn't have to copy/paste code to manually polyfill this when needed.

Might want to recommend installing it too in the README