Migrate ESGST to Manifest v3

[Eiion]

With Google letting Manifest v2 expire since early June and is rolling Manifest v3 out to replace it to more and more users, it's not unlikely that ESGST will stop working completely.

It appears as though ESGST is now an abandoned extension for over two years now and the change of the extension to Manifest v3 will not happen if Rafael isn't making the change. Maybe the userscript versions would still work - we'll see.

Rafael, after many years where you gave us all great support and put in a lot of time and effort, after having helped with the script and later the extension within the very limited extent of my abilities several years ago, I'm very thankful for what you have given us and we will accept if you have decided to not spend any more time with this great tool. Thanks for your contribution to the community - I hope you are well.