Extensions: Convert the various creators into a single creator

[aalves08]

Is your feature request related to a problem? Please describe.

Convert the various creators into a single creator package/lib.

We currently have:
skeleton app creation: https://github.com/rancher/dashboard/tree/master/shell/creators/app
extension/package creation: https://github.com/rancher/dashboard/tree/master/shell/creators/pkg
extension update: https://github.com/rancher/dashboard/tree/master/shell/creators/update

Describe the solution you'd like

Describe alternatives you've considered

Additional context

Suggestion:

create a new folder for your code
yarn create @rancher/extension EXTENSION_NAME

Operates a bit more intelligently.

So looks to see if there is an app, if not, it creates one then it creates the extension.
If there is, it adds the extension.
app name and extension name would be the same - maybe an arg to give a different app name
Maybe an arg to only create an app

[izaac]

@yonasberhe23 this seems to be only configuration updates and tooling, looks good to close to me. Thoughts?

[izaac]

Moving to done