snapcraft extension command does not exist

[waveform80]

Bug Description

The snapcraft documentation states:

Further information about any specific extension can be obtained by typing snapcraft extension followed by the extension name:

$ snapcraft extension gnome-3-28
This extension eases creation of snaps that integrate with GNOME 3.28
...

But this sub-command is missing from snapcraft 8.

To Reproduce

$ snapcraft --version
snapcraft 8.4.3                                                                                         
$ snapcraft extensions | grep kde-neon
kde-neon                core18, core20, core22
kde-neon-6              core22
$ snapcraft extension kde-neon-6
Usage: snapcraft [options] command [args]...
Try 'snapcraft -h' for help.

Error: no such command 'extension', maybe you meant 'extensions', 'list-extensions' or 'expand-extensions'

Environment

Ubuntu 24.04 LTS, snapcraft 8.4.3

snapcraft.yaml

n/a

Relevant log output

See reproduction section above.

Additional context

n/a

[mr-cal]

@cmatsuoka or @sergiusens - do we still want to support this command in snapcraft?