New grand scripting API overhaul

The mpv hook notation has more than overstayed its welcome. Proposals for new scripting APIs:

1. libplacebo's own native C plugin API (.so/.dll + dlopen). This one is very easy to support, and hugely important, since it allows us to use e.g. ncnn or similar libraries.
2. Something based on javascript, via duktape or mujs.
3. Support for slang shader system.
4. Something based on lua, via luajit or w/e

Currently leaning towards 1 > 2 > 3. 1 will definitely happen, 2 might happen. Not a fan of the lua ecosystem, so 4 is probably out. 3 is an interesting opportunity to explore. It seems like a lot of effort, but it could be done. (Is there some sort of C library for dealing with these files, that we could integrate?)