Move tasks to individual files

[NathanHB]

getting rid of the default_tasks.py file containing 1000s tasks and putting all of them into separate files. This allows for:

• better maintenance
• more readability (one task one file)
• adding metadata for every tasks

Also, getting rid of many duplicate tasks, for example, the same task in 2 different suites. I'm only keeping/moving all tasks to the lighteval suite so that we can eventually get rid of the suite system.

• move tasks from default_tasks
• move extended tasks
• move multilingual tasks
• move community tasks
• add docstring for default tasks
• add docstring for multilingual tasks
• add docstring for community tasks
• make a space to explore all the tasks, based on the tasks docstrings.

We can then use the metadata and the one file system to build a viewer for all the tasks: