Notify Nix users that extension directory must be mutable

[sgoudham]

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:

vscode/packages/catppuccin-vsc/src/utils.ts

Lines 71 to 77 in f0daac5

	// TODO: listen to this to determine if a user is using Nix, redirect to README
	export const isMutable = async (uri: Uri): Promise<boolean> => {
	return workspace.fs.stat(uri).then(
	(stat) => stat.permissions !== FilePermission.Readonly,
	(error) => error,
	);
	};

---

Arises from #415

[sgoudham]

I tried implementing this earlier and it was an absolute pain to debug. I thought I'd be able to mimic the error on a non nix setup by setting the files to read only but it seems more complicated than that as I wasn't succeeding in getting the popup to show up when I wanted it to.