[5.7] Allow storage assertions to handle multiple files at once

[freekmurze]

This allows assertExists and assertMissing to handle multiple files at once. It also makes them chainable.

Storage::disk('local')
    ->assertExists([
        $path,
        $anotherPath,
    ])
    ->assertMissing([
        $yetAnotherPath,
        $oneMoreFilePath,
    ]);

There are no existing tests for the assertions, so I didn't add any for this new functionality.