Notify Nix users that extension directory must be mutable

Some users manage their VSCode via NixOS/NixOS Home Manager which results in the extension directory being immutable. As stated in the README, the extension makes the assumption that the extension directory is mutable in order to configure the theme via the options provided.

A `isMutable` function has already been implemented which we will want to build off of to notify Nix users that the advice in the README should be followed:

https://github.com/catppuccin/vscode/blob/f0daac51ddc194a3d91ec07277bd79c6168870f5/packages/catppuccin-vsc/src/utils.ts#L71-L77

Arises from #415

Start with the existing isMutable function in packages/catppuccin-vsc/src/utils.ts at the linked lines, then read the README guidance about mutable extension directories. Add a notification for users whose extension directory is immutable, directing Nix users to follow that README advice. Confirm the notification appears for the immutable case without changing the mutable-case behavior.

Written by the indexing model from the issue text.