extension-tool: Provide alternative extension templates
Something that came up in World/ShellExtensions/extensions-rebooted#1:
The template used when creating a new extension is intentionally
minimal, as the sample code in the old extensions-tool proved to
be annoying more often than not.
However as we support per-command options, we don't have to limit
ourselves to a single template, and can offer alternatives for
common use cases.
label 5. Extensions label 2. RFC