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>

scripts/docker.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+# Runs a command in the development image, in this folder, as you (so the
+# files it makes are yours): ./scripts/docker.sh npm run package
+# Without a command: a shell. The image is (re)built when the Dockerfile changes.
+set -e
+root=$(cd "$(dirname "$0")/.." && pwd)
+image=vs-code-cmake-helper-dev
+docker build -q -t "$image" "$root" >/dev/null
+tty=; [ -t 0 ] && tty=-it
+exec docker run --rm $tty -u "$(id -u):$(id -g)" -e HOME=/tmp -v "$root:/workspace" -w /workspace "$image" "${@:-bash}"