Multiple bugs in assets:generate

Bug: `[LogicException] An option named "env" already exists`
Removed the env option and added environment option exists check.

Bug: `Class 'Codesleeve\\Sprockets\\StaticFileGenerator' not found"`
`StaticFileGenerator` should be `StaticFilesGenerator`

Bug: `Undefined index: routing.prefix`
`$config['routing.prefix']` changed to `$config['routing']['prefix']`



##### Additional bugs uncovered:
`assets:generate` does not concat files or process directives in application.js/css. Resulting in environments in `concat` config linking to a single, empty application.css/js file. Environments excluded in `concat` are linked to the individual css/js files as expected. 

application.css/js files are empty no matter the `assets:generate` environment.

`assets:clean` throws:

```
{
   "error":{
      "type":"Symfony\\Component\\Debug\\Exception\\FatalErrorException",
      "message":"Cannot access private property Codesleeve\\AssetPipeline\\AssetPipeline::$parser",
      "file":"C:\\xampp\\htdocs\\brands\\vendor\\codesleeve\\asset-pipeline\\src\\Codesleeve\\AssetPipeline\\Commands\\AssetsCleanCommand.php",
      "line":74
   }
}
```