llama : add enum for built-in chat templates

[ngxson]

Add enum llm_chat_template to enumerate all built-in chat templates. This can help generating documentation:

--chat-template JINJA_TEMPLATE          set custom jinja chat template (default: template taken from model's
                                        metadata)
                                        if suffix/prefix are specified, template will be disabled
                                        list of built-in templates:
                                        chatglm3, chatglm4, chatml, command-r, deepseek, deepseek2, exaone3,
                                        gemma, granite, llama2, llama2-sys, llama2-sys-bos, llama2-sys-strip,
                                        llama3, minicpm, mistral-v1, mistral-v3, mistral-v3-tekken,
                                        mistral-v7, monarch, openchat, orion, phi3, rwkv-world, vicuna,
                                        vicuna-orca, zephyr
                                        (env: LLAMA_ARG_CHAT_TEMPLATE)