This commit adds completion for `--chat-template-file`, enabling only
`.jinja` files to be displayed as completions.
Example usage:
```console
$ ./build/bin/llama-cli --chat-template-file models/templates/<TAB>
models/templates/CohereForAI-c4ai-command-r7b-12-2024-tool_use.jinja
models/templates/CohereForAI-c4ai-command-r-plus-tool_use.jinja
models/templates/deepseek-ai-DeepSeek-R1-Distill-Llama-8B.jinja
models/templates/deepseek-ai-DeepSeek-R1-Distill-Qwen-32B.jinja
models/templates/fireworks-ai-llama-3-firefunction-v2.jinja
models/templates/google-gemma-2-2b-it.jinja
models/templates/llama-cpp-deepseek-r1.jinja
models/templates/meetkai-functionary-medium-v3.1.jinja
models/templates/meetkai-functionary-medium-v3.2.jinja
models/templates/meta-llama-Llama-3.1-8B-Instruct.jinja
models/templates/meta-llama-Llama-3.2-3B-Instruct.jinja
models/templates/meta-llama-Llama-3.3-70B-Instruct.jinja
models/templates/microsoft-Phi-3.5-mini-instruct.jinja
models/templates/mistralai-Mistral-Nemo-Instruct-2407.jinja
models/templates/NousResearch-Hermes-2-Pro-Llama-3-8B-tool_use.jinja
models/templates/NousResearch-Hermes-3-Llama-3.1-8B-tool_use.jinja
models/templates/Qwen-Qwen2.5-7B-Instruct.jinja
```
This is not limited to the models/templates directory, it can be used
anywhere in the filesystem, the above is just an example.
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