Allow for explicit output filetype selection

[alexszeliga]

In my use case, I want to stream my export to a download without creating a file, so I use the PHP write only output stream, php://output, but that limits me to using xlsx, with no way to stream csv.

I changed the signature to the download and export methods to allow for an optional extension. When provided, it allows the developer to explicitly select any file type, regardless of filename.

[alexszeliga]

I simplified the complexity issue and removed a Laravel Str helper dependency, since PHP 8 has semantic methods to replace them.