You can find the GitHub actions stuff in the jenkins-std-lib https://github.com/DontShaveTheYak/jenkins-std-lib it has both javadoc documentation or there is a jobs folder with examples.

For groovy auto-completion I have the vscode plugin groovy-guru. https://github.com/DontShaveTheYak/groovy-guru

I also have the Jenkins extension pack for vscode https://github.com/DontShaveTheYak/jenkins-extension-pack

And in a few weeks at the Jenkins contributor summit I'll be giving a presentation on developing pipelines with vscode.