Add Backdrop CMS installer

`composer/installers` has installers for many CMSes (including Drupal) but none for **Backdrop CMS**, a fork of Drupal.

Backdrop contrib is distributed as `backdrop-module`, `backdrop-theme`, and `backdrop-layout` packages. Without a Backdrop installer, these install into `vendor/` instead of the site's docroot (`modules/contrib/`, `themes/contrib/`, `layouts/contrib/`), so Backdrop sites can't manage contrib with Composer without a third-party extender.

Proposed mapping (standard Backdrop layout):

| type | path |
|---|---|
| `backdrop-core` | `core/` |
| `backdrop-module` | `modules/{$name}/` |
| `backdrop-theme` | `themes/{$name}/` |
| `backdrop-layout` | `layouts/{$name}/` |
| `backdrop-profile` | `profiles/{$name}/` |
| `backdrop-drush` | `drush/{$name}/` |

Start by comparing the existing Drupal installer with the proposed Backdrop package types and paths. Check how the project verifies installer mappings, then confirm each listed type installs to its specified path.

Written by the indexing model from the issue text.