Add support for dev container lockfile maintenance

[sisp]

Tell us more.

Dev containers support an optional devcontainer-lock.json lockfile. VS Code v1.118 enables this feature by default, so many projects using dev containers and VS Code will start seeing this lockfile unless explicitly disabled. AFAIK, Renovate doesn't support lockfile maintenance of devcontainer-lock.json yet, hence I'm suggesting to add this capability.

[ferferga]

The lockfile should not only be updated with lockfile maintenance, but also when a feature is updated. Right now the lockfile is never updated.