Add subsets for lcb

[plaguss]

Description

Adds subsets to lcb:

The previous subset v4_v5 keeps with the same name to avoid confusion, but now 2 more subsets are included:

MODEL_ARGS=...
lighteval vllm \
    $MODEL_ARGS \
    "extended|lcb:codegeneration_v4|0|0" \
    --use-chat-template

and

lighteval vllm \
    $MODEL_ARGS \
    "extended|lcb:codegeneration_v5|0|0" \
    --use-chat-template

@edbeeching, @lewtun, @NathanHB

[HuggingFaceDocBuilderDev]

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[lewtun]

Thanks for the additional subsets! LGTM if @NathanHB also approves