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/npm/.npmrc

@@ -0,0 +1,2 @@
+min-release-age=7 ; 7 days for npm
+minimum-release-age=10080 ; 7 days in minutes for pnpm