Revisit Extension creator mechnisim

[jordojordo]

Follow on to #11484

Is your feature request related to a problem? Please describe.
There were a few changes requested in #11605, they should be addressed.

Previous behaviour is that the creator will create etc using the shell version of the creator.
For the update behaviour, the idea being that the logic to do the update was in the creator with the same version as the shell.
Originally posted by @nwmac in #11605 (comment)

How does a user create a new extension with a specific shell version?
Originally posted by @nwmac in #11605 (comment)

Seem to have lost the formatting we use with out lint rules
Originally posted by @nwmac in #11605 (comment)

Can we call this 'my-extension' rather than 'my-app' ?
Originally posted by @nwmac in #11605 (comment)