Enable use of RenderedContentInterface and small fix

[mallardduck]

hey @freekmurze - this change makes a quick fix for the 2.0 release related to extensions to help ensure things work properly. There were parts of the initial changes I made that were not covered by tests to catch the logic issue.

Quickfix

Make sure extensions can be given as class names (strings) or instances. So i've made adjustments to ensure extension load properly.

Specifically these lines would be affected:

laravel-markdown/src/MarkdownRenderer.php

Line 149 in 991ff79

$environment->addExtension(new $extension());

laravel-markdown/src/MarkdownRenderer.php

Line 78 in 991ff79

public function addExtension(ExtensionInterface $extension): self

New feature

This also adds a method that mimics the league convertToHtml method for users. Adding this method and covering it with tests was what caused me to encounter the error to begin with. Otherwise I would have opted to split them up.

This addition is important for users that want the FrontMatterExtension (or similar). Since these shouldn't return a string directly like existing methods do. So the new method returns a RenderedContentInterface object specific to the renderer. For instance, they can use any potential extension specific results. Like:

$result = app(\Spatie\LaravelMarkdown\MarkdownRenderer::class)->renderMarkdown($markdown);

// Grab the front matter:
if ($result instanceof RenderedContentWithFrontMatter) {
    $frontMatter = $result->getFrontMatter();
}

[freekmurze]

Thank you very much!