refactor: group stow packages into common, mac and kde

Split the packages by platform so each app stays an opt-in Stow package
under common/, mac/ or kde/. KDE gets its own Fastfetch, Helium and
Spectacle packages, and darwin-breeze becomes kde/theme. The READMEs
document stowing a whole group as well as individual packages.

common/zsh/.zshenv

@@ -0,0 +1,7 @@
+export XDG_CONFIG_HOME="${XDG_CONFIG_HOME:-$HOME/.config}"
+export XDG_DATA_HOME="${XDG_DATA_HOME:-$HOME/.local/share}"
+export XDG_CACHE_HOME="${XDG_CACHE_HOME:-$HOME/.cache}"
+export XDG_STATE_HOME="${XDG_STATE_HOME:-$HOME/.local/state}"
+
+XDG_BIN_HOME="${XDG_BIN_HOME:-$HOME/.local/bin}"
+ZDOTDIR="$XDG_CONFIG_HOME/zsh"