plugins/poimandres: add colorscheme + test

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

@@ -0,0 +1,54 @@
+{
+  # Empty configuration
+  empty = {
+    colorschemes.poimandres.enable = true;
+  };
+
+  # All the upstream default options of poimandres
+  defaults = {
+    colorschemes.poimandres = {
+      enable = true;
+
+      boldVertSplit = false;
+      darkVariant = "main";
+      disableBackground = false;
+      disableFloatBackground = false;
+      disableItalics = false;
+      dimNcBackground = false;
+
+      groups = {
+        background = "background2";
+        panel = "background3";
+        border = "background3";
+        comment = "blueGray3";
+        link = "blue3";
+        punctuation = "blue3";
+
+        error = "pink3";
+        hint = "blue1";
+        info = "blue3";
+        warn = "yellow";
+
+        git_add = "teal1";
+        git_change = "blue2";
+        git_delete = "pink3";
+        git_dirty = "blue4";
+        git_ignore = "blueGray1";
+        git_merge = "blue2";
+        git_rename = "teal3";
+        git_stage = "blue1";
+        git_text = "teal2";
+
+        headings = {
+          h1 = "teal2";
+          h2 = "yellow";
+          h3 = "pink3";
+          h4 = "pink2";
+          h5 = "blue1";
+          h6 = "blue2";
+        };
+      };
+      highlightGroups = {};
+    };
+  };
+}