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>

package.json

@@ -377,7 +377,7 @@
         },
         {
           "command": "cmakeHelper.debug",
-          "when": "view == cmakeHelper.targets && viewItem =~ /^target\\.executable|\\.run$/",
+          "when": "view == cmakeHelper.targets && viewItem =~ /^target\\.(executable|utility)|\\.run$/",
           "group": "inline@2"
         },
         {
@@ -397,7 +397,7 @@
         },
         {
           "command": "cmakeHelper.debug",
-          "when": "view == cmakeHelper.targets && viewItem =~ /^target\\.executable|\\.run$/",
+          "when": "view == cmakeHelper.targets && viewItem =~ /^target\\.(executable|utility)|\\.run$/",
           "group": "2_run@2"
         },
         {