Generates starter Incant admin files and patches Phoenix setup when possible.
The installer creates:
lib/my_app/admin.exlib/my_app/admin/resources/sample.exlib/my_app/admin/themes/default.ex
When it can detect standard Phoenix files, it also adds use Incant.Router,
mounts incant "/admin", MyApp.Admin, and adds the Incant Tailwind
source to assets/css/app.css.
Use Igniter's regular options such as --dry-run and --yes to preview or
apply changes.