split default tasks from core

[tharvik]

currently, most tasks are hardcoded inside discojs-core, which looks to me as out of place: it kinda encourage people to expect that it will always contains theses tasks thus directly depending on it & it also mixes where the tests of theses tasks are with the core code (it is kinda done in server eventhough it's not really the correct place to do it).

splitting theses in a new package, @epfml/discojs-tasks would allow for easier adding/modifing/removing of tasks, and have a dedicated place for testing theses. we would also be able to store raw data (such as image or models) directly there.

[JulienVig]

Before refactoring the tasks I think it is important for us to define what's the purpose of tasks in Disco and what they are, especially what are the use cases for pre-defined tasks and custom tasks. And how LLM use would fit in those.