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>

example/tests/greet_test.cpp

@@ -0,0 +1,10 @@
+#include <iostream>
+#include "greet.h"
+
+int main() {
+  if (greet("CMake") != "Hello, CMake!") {
+    std::cerr << "unexpected greeting: " << greet("CMake") << std::endl;
+    return 1;
+  }
+  return 0;
+}