feat(theming): add theme discovery with gruvbox assets

internal/theming/assets/gruvbox/zellij.kdl

@@ -0,0 +1,35 @@
+theme "gruvbox-dark"
+default_layout "default"
+default_mode "normal"
+pane_frames false
+simplified_ui true
+mouse_mode false
+
+keybinds {
+    shared {
+        bind "Alt h" { MoveFocusOrTab "Left"; }
+        bind "Alt l" { MoveFocusOrTab "Right"; }
+        bind "Alt j" { MoveFocus "Down"; }
+        bind "Alt k" { MoveFocus "Up"; }
+        bind "Alt n" { NewPane "Down"; }
+        bind "Alt v" { NewPane "Right"; }
+        bind "Alt f" { ToggleFloatingPanes; }
+        bind "Alt t" { NewTab; }
+    }
+}
+
+themes {
+    gruvbox-dark {
+        fg "#ebdbb2"
+        bg "#282828"
+        black "#1d2021"
+        red "#cc241d"
+        green "#98971a"
+        yellow "#d79921"
+        blue "#458588"
+        magenta "#b16286"
+        cyan "#689d6a"
+        white "#a89984"
+        orange "#d65d0e"
+    }
+}