local LLM -- option to select *.gguf models by myself

Hello.

I am happy with this extension. It is the only one, that works without any dependencies and therefore it is instantly usable within VScode. Thank you for bringing us such an extension. I have **some requests**:

It would be nice, if we could have a option to select the *.gguf model file by our own. I can download it somewhere (f.e. huggingFace) and inside the options of FireCoder i can select the file. I recognized that the models are saved inside C:\ drive. inside the USERS subfolder: ".firecoder". If its possible it would also be very handy, if we could have an option where to store downloaded models.

In my case, the drive C: is reserved for the OS and stays clean. I put heavy stuff onto the D: partition. (Windows OS). I managed to use the MkLink command, to alias this folder to drive D:.

未指定文件、测试或入口点。首先定位 VS Code 设置以及现有的模型下载/存储流程，然后确定用户选择的 .gguf 文件和可配置的模型目录应如何结合；当所请求的两个选项都能正常工作且不破坏当前的模型行为时，即视为完成。

由索引模型根据 Issue 内容生成。