Align generator invocation

[tsmaeder]

We have one yeoman generator that is invoked with yo @theia/plugin and this one is invoked with yo theia-extension. Why not use the same syntax like yo @theia/extension?

[JonasHelming]

Not sure about the @theia/extension name though. interested in a second opinion @msujew

[msujew]

Ideally, I would just use theia, given that the yeoman generator essentially sets up a whole Theia based project and not just an extension. I believe the generator-theia package on npm can be re-used for that.

[JonasHelming]

As a very first thing, I will publish the existing package again under the name "theia-generator". The existing theia-generator is 7 years old, but downloaded 6 times per week and likely confuses people. @msujew Do you see any issues with this?
We can then fully migrate to the new name.