Migrating to Cheerpj 3

[oeway]

It seems there is a more poweful java-to-js compiler coming: https://leaningtech.com/announcing-cheerpj-3-0-a-jvm-replacement-in-html5-and-webassembly-to-run-java-applications-and-applets-on-modern-browsers/

From the description, there seems to be a lot of improvements with the new architecture based on WebAssembly. Now they can ship an entire JDK, and there seems to be multithreading support, which is exciting!

cc @ctrueden maybe you can comment on what's it means for imagej on the web? Would it make it easier for porting imagej2 based on their description?