fix(bigbench): add group for task discovery (bigbench)

[jayvenn21]

Fix BIG-bench task discovery

Some BIG-bench tasks exist but are not discoverable under the name bigbench,
causing lm_eval ls tasks and --tasks bigbench to fail with "Tasks not found".

This PR:

• Adds a group config _bigbench.yaml with group: bigbench listing all 286 BIG-bench subtasks
• Enables --tasks bigbench and makes "bigbench" appear in lm_eval ls tasks
• Preserves backward compatibility (no file renames, no task renames)
• Task list is derived from existing YAMLs under lm_eval/tasks/bigbench/

Fixes #3458