plugins/catppuccin: add colorscheme + test

tests/test-sources/plugins/colorschemes/catppuccin.nix

@@ -0,0 +1,50 @@
+{
+  # Empty configuration
+  empty = {colorschemes.catppuccin.enable = true;};
+
+  # All the upstream default options of poimandres
+  defaults = {
+    colorschemes.catppuccin = {
+      enable = true;
+      flavour = "mocha";
+      background = {
+        light = "latte";
+        dark = "mocha";
+      };
+      transparentBackground = false;
+      terminalColors = true;
+      showBufferEnd = false;
+      dimInactive = {
+        enabled = true;
+        shade = "dark";
+        percentage = 0.15;
+      };
+      disableItalic = true;
+      disableBold = true;
+      styles = {
+        comments = ["italic"];
+        conditionals = ["italic"];
+        loops = [];
+        functions = [];
+        keywords = [];
+        strings = [];
+        variables = [];
+        numbers = [];
+        booleans = [];
+        properties = [];
+        types = [];
+        operators = [];
+      };
+      colorOverrides = {};
+      customHighlights = {};
+      integrations = {
+        cmp = true;
+        gitsigns = true;
+        nvimtree = true;
+        telescope = true;
+        notify = false;
+        mini = false;
+      };
+    };
+  };
+}