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.

kde/theme/vscode/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "darwin-breeze",
+  "displayName": "Darwin Breeze",
+  "description": "Generated by kde/theme/build.py from DarwinBreezeDark.colors",
+  "publisher": "trivk",
+  "version": "1.0.0",
+  "engines": {
+    "vscode": "^1.70.0"
+  },
+  "categories": [
+    "Themes"
+  ],
+  "contributes": {
+    "themes": [
+      {
+        "label": "Darwin Breeze Dark",
+        "uiTheme": "vs-dark",
+        "path": "./themes/DarwinBreezeDark-color-theme.json"
+      }
+    ]
+  }
+}