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:.

未指定檔案、測試或進入點。先找出 VS Code 設定與現有的模型下載／儲存流程，接著確認使用者選取的 .gguf 檔案與可設定的模型目錄應如何配合；當要求的兩個選項都能正常運作，且不會破壞目前的模型行為時，即表示完成。

由索引模型根據 Issue 內容生成。