dmenu: the package, which makes the notifier's menu work, and a menu tool (hq to-be 42 phase 2.7)

Research 026/04 counted two plain dmenu calls failing with dmenu installed
nowhere. Measured, they are one line on each workstation: dunst's
`dmenu = /usr/bin/dmenu -p dunst:`. Installing the package fixes both by
existing; the line stays the dunst module's.

No claim: node-launcher is not in the controller's seat table yet, and the
module says so. Two Go tools: menu, shaped like that seat's verb (chosen
line, index, typed, cancelled, timed out within 25 s), and session.

modules/dmenu/module.json

@@ -0,0 +1,33 @@
+{
+  "module": "dmenu",
+  "version": "1",
+  "capabilities": [
+    "package-manager"
+  ],
+  "tools": [
+    "dmenu_menu",
+    "dmenu_session"
+  ],
+  "resources": [
+    {
+      "id": "package",
+      "type": "package",
+      "package": "dmenu"
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "tools",
+        "kind": "bundle",
+        "language": "go",
+        "system": "arch",
+        "from": "cmd/dmenu-tools",
+        "binary": "dmenu-tools",
+        "loads": [
+          "dmenu-tools"
+        ]
+      }
+    ]
+  }
+}