Debug custom targets, and a Docker image to build and develop in

- 🐞 on a custom target without an executable set debugs the program its
  command runs, as the build system has it (Ninja's edge, a Makefile's
  recipe): its arguments, directory and cmake -E env variables; what the
  target depends on is built first, not the target itself
- Debug for custom targets in the status bar, the Targets view and the
  code lenses above add_custom_target
- Dockerfile (Node, CMake, Ninja, g++, GDB), scripts/docker.sh to run a
  command in it, a dev container, and example/ to try the extension on

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -0,0 +1,14 @@
+{
+  "name": "VS Code CMake Helper",
+  "build": { "dockerfile": "../Dockerfile", "context": ".." },
+  "workspaceFolder": "/workspace",
+  "workspaceMount": "source=${localWorkspaceFolder},target=/workspace,type=bind",
+  "remoteUser": "node",
+  "postCreateCommand": "npm ci",
+  "customizations": {
+    "vscode": {
+      // C/C++ for example/, and the debugger the extension's Debug uses.
+      "extensions": ["ms-vscode.cpptools"]
+    }
+  }
+}