Add installer types associated with Asset Packagist

## Summary
Asset Packagist, https://asset-packagist.org/ , is an endpoint that allows sites to retrieve NPM and Bower libraries via Composer. Its assets have two defined installer types, `npm-asset` and `bower-asset`.

With the recent (May 2026) abandonment of https://packagist.org/packages/oomphinc/composer-installers-extender , setups that currently used that Composer plugin's "installer-types" method to register these installer types can no longer do so.

As such, it seems appropriate for Composer Installers to incorporate the `npm-asset` and `bower-asset` installer types.

## This was previously requested
This has been requested twice before, in https://github.com/composer/installers/issues/394 and https://github.com/composer/installers/issues/513 . However, the abandonment of `composer-installers-extender` provides a reason for the maintainers of Composer Installers to reconsider this request.

## Proposed resolution
Add an `AssetPackagistInstaller.php` as follows:

```php
*/
protected $locations = array(
'npm-asset' => 'libraries/{$name}/',
'bower-asset' => 'libraries/{$name}/',
}
```