Fix adding classname when using SyntaxHighlighter block

[renatho]

Fixes #123 and fixes #130

Changes proposed in this Pull Request:

• Fix adding classname when using Syntax Highlighter block

Testing instructions:

• Create a new Syntax Highlighter block in a post or page.
• In the sidebar, Advanced → Additional CSS class(es) add a class.
• Open the page on the frontend and check if the class was added.

[gikaragia]

So after testing this a bit I have found out that only the first class is included in the frontend.

The HTML that the plugin generates it is correct (it includes all classes) however the SyntaxHighlighter JS library modifies the HTML and it only adds the first class. It's supposed to support multiple classes however I wasn't able to make it work and reading its source code didn't help.

I think that we should merge this fix but leave issue #130 open, lower its priority and repurpose it to mention that multiple classes are not supported. Not sure if a fix is possible without modifying the dependency library though.

[renatho]

So after testing this a bit I have found out that only the first class is included in the frontend.

Good catch!

I realized here that he uses single quotes in the class-name attribute. So I tested and it worked with multiple classes \o/

Fixed here: 53786a3

The name of the method where I fixed it is shortcode_callback, but the render_block also calls this method. Alex leave this comment in the render_block method:

Renders the content of the Gutenberg block on the front end using the shortcode callback. This ensures one source of truth and allows for forward compatibility.

[gikaragia]

Nice! Didn't thought of using quotes :)