Improve the QuickPick for choosing extensions

[xorye]

Currently, the QuickPick for choosing extensions looks like the following:

It would be nice if the "RESTEasy JAX-RS" entry was always checked because RESTEasy JAX-RS is included in all Quarkus projects by default. Currently, the user can check it, or uncheck it. But regardless of if its checked or not, that extension is always included.

Also, like https://code.quarkus.io/, it would be nice if there was a "Preview" label for the preview extensions. This information is provided by the code.quarkus.io /api/extensions endpoint,

Lastly, having predefined categories for the extensions list would also be a nice touch.

[xorye]

@fbricon Instead of having the "RESTEasy JAX-RS" entry being always checked, would it be better to just remove that entry completely?

Because if we leave it in, it might look a bit bloaty: