Figure out the minimum browser requirements

[guregu]

This project is using a lot of bleeding edge WebAssembly stuff so it does not have good compatibility with old browsers.
I am mostly interested in supporting evergreen browsers (the latest versions) but I would like to support older ones if it's possible without too much pain.

Current situation:

• Safari: needs 15 for Uint64Array
• Firefox: needs 78 for uint64 support

I would also like to use top-level await which pushes Firefox to 89, but I am not sure which tooling that might break so I'll hold off on it for now. With top-level await, we can remove the necessity for the load() function and let people call new Prolog() as soon as the import resolves.

[guregu]

Also worth mentioning, we use FinalizationRegistry if we can. This is from Firefox 79. If we start using top-level await we can remove the compatibility check here.
This functionality isn't strictly necessary but it can help clean up memory from leaked queries.