Add `ligheval --list-tasks`

[clefourrier]

This command should list all the available tasks (in extended tasks and the task table)

[DimbyTa]

Hello,

I would like to work on this. Do you need special formatting for the list?

[clefourrier]

Hi, thanks for your interest! I think providing the tasks grouped by suite would be nice, for example

- lighteval: all the tasks available, sorted alphabetically
- community: all the tasks available, sorted alphabetically
...

[NathanHB]

We have a lot of tasks so it would be better to have them in a list in that format, this makes reading easier as you can ligheval --list-tasks | grep mmlu for example.

- lighteval:
    - task 1
    - task 2
- community:
    - task 1 
    - task 2

[DimbyTa]

I understand. I'm working on it now.