small improvements & security update

changelog:
- added additional commands for nixos `gc` in fish
- changed default app to `mpv` for gif images in mimeapps.list
- added config for clamav-scanner
- added `fangfrisch` to clamav
- sandboxed some apps & added profiles through firejail
- changed vscode to vscodium
- updated README

nixos/clamav-scanner.nix

@@ -0,0 +1,8 @@
+{ ... }:
+
+{
+  services.clamav.scanner = {
+    enable = true;
+    interval = "Sat *-*-* 04:00:00";
+  };
+}