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>

Dockerfile

@@ -0,0 +1,13 @@
+# Everything the extension needs to be built and packaged (Node), and what it
+# drives, to try it on example/: CMake, Ninja, a C++ compiler, GDB.
+# Used by scripts/docker.sh and by the dev container (.devcontainer/).
+FROM node:22-bookworm
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends cmake ninja-build make g++ gdb gdbserver git ca-certificates \
+ && rm -rf /var/lib/apt/lists/*
+
+ENV NPM_CONFIG_UPDATE_NOTIFIER=false
+WORKDIR /workspace
+USER node
+CMD ["bash"]