Making of LLemMings

Persist progress in local storage

ChatGPT: prompt below

Minor other human tweaks were done:
- added a runOnce() function to load persisted data on page-load
- instead of having entry point in the HTML page, it is now runOnce()
- added faster testing for completing level 1 as it gets pretty boring
to wait for it when testing this

>>> Prompt:
Given client-side javascript and this snippet:
let persisted = {
currentLevel : 1,
currentLevelAttempts : 0,
overallScore : 0,

I want functions for serializing and deserializing the contents of that variable
in local storage. All code should be in aptly named functions.

This time I don't need explanation, just the code.