[11.x] Add Support for Extensions in Str::markdown Method

[tnylea]

Hey Taylor and team 👋 Hope you guys are doing well!

I wanted to submit a simple PR that allows anyone using the Str::markdown(...) method to add some CommonMark Extensions as a third parameter. Here is a quick example of how you could use this in a controller:

public function parseMarkdownFromFile($file_location){

    $markdown_contents = File::get($file_location);
    
    $html = Str::markdown($markdown_contents, [], [
        new AttributesExtension(),
        new TaskListExtension(),
    ]);

    return $html;

}

Here's a quick 60 second video of this in action 👍 Sound on 🔉

common-mark-extensions.mp4

I didn't see any tests associated with the Str::markdown() method, but if there are some you would like me to update, let me know 😉 I'm open to making any changes if there's another way you want to see this implemented.

Really appreciate it. Talk to you soon!

[shaedrich]

The environment is only needed when $extensions !== []. Wouldn't it make sense, to wrap this in an if?