llama : add completion for --chat-template-file (ggml-org#11860)

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.

Author: danbev

common/arg.cpp

@@ -409,6 +409,10 @@ static void common_params_print_completion(common_params_context & ctx_arg) {
     printf("            COMPREPLY=( $(compgen -f -X '!*.gbnf' -- \"$cur\") $(compgen -d -- \"$cur\") )\n");
     printf("            return 0\n");
     printf("            ;;\n");
+    printf("        --chat-template-file)\n");
+    printf("            COMPREPLY=( $(compgen -f -X '!*.jinja' -- \"$cur\") $(compgen -d -- \"$cur\") )\n");
+    printf("            return 0\n");
+    printf("            ;;\n");
     printf("        *)\n");
     printf("            COMPREPLY=( $(compgen -W \"${opts}\" -- \"$cur\") )\n");
     printf("            return 0\n");