vscodium: wire through HM programs.vscode so stylix can theme it

Stylix's vscode target injects workbench.colorCustomizations via the
HM programs.vscode module — it can't reach a system-package install.
Move vscodium to programs.vscode with mutableExtensionsDir = true so
GUI-installed extensions stay put, and preserve the six user settings
that aren't colour-related.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

hosts/FredOS-Gaming.nix

@@ -22,7 +22,6 @@
       adwsteamgtk
       protonvpn-gui
       onlyoffice-desktopeditors
-      vscodium
       woeusb
     ];