Automatically disable copilot in dotenv files

It seems like a big security issue to have Copilot running by default within dotenv files (i.e. `.env`) that contain extremely sensitive information (API keys, access tokens, passwords, etc).

Can this be disabled by default?

I've circumvented this locally with this code in my vimrc:

```vimscript
augroup dotenv_detect
autocmd!
autocmd BufRead,BufNewFile *.env,*.env.* set filetype=dotenv
augroup END

let g:copilot_filetypes = {
\ '*': v:true,
\ 'dotenv': v:false
\ }
```

But this feels like a pretty general best practice that should be opt-out, not opt-in.

以 vimrc workaround 作為行為參考：對於 .env 和 .env.* 等 dotenv 檔案，Copilot 預設應處於停用狀態。先追蹤 filetypes 的偵測方式以及 g:copilot_filetypes 的套用方式，然後確認一般的 filetypes 仍保持啟用，而 dotenv 檔案預設處於停用狀態。

由索引模型根據 Issue 內容生成。