Consider adding flag to extensions to list experimental extensions

[hartmutobendorf]

What needs to get done

add --all flag to list-extensions to show all (including experimental) add information about how to use experimental extensions to the output list (as in the docs, see below):

To enable the use of experimental extensions in Snapcraft, set the environment variable SNAPCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS to 1.

Why it needs to get done

The rationale for adding this would be: Users should be able to see experimental extensions without having to go to documentation.