Code block: add language support to retain language information when pasting markdown

Describe the bug When I paste markdown, I like/expect that the language specified after the start of a backtick fence will be converted into a CSS class about the language specified. So if I start the block like ```php I expect to see a <code class="language-php"> in the block. This isn't happening.

Here's a CommonMark spec/test about the behavior that matches my expectations: https://spec.commonmark.org/0.28/#example-111

To Reproduce Steps to reproduce the behavior:

1. Open Gutenberg editor
2. Paste code like

```php
<?php

$random = 'what';
```

3. See that there is not a "language-php" class applied to that code block by default.

Expected behavior That the language-php or similar is honored and applied as a class to either the internal code block (like the CommonMark spec) or even to the outer pre block.

Desktop (please complete the following information):

• OS: macOS
• Browser: Firefox
• Version 62.0.2

Additional context

• Noticed while using Gutenberg 3.9
• I know there are lots of "dialects" of Markdown. And I'm confident that both the CommonMark standard and "GitHub Flavored Markdown" (link about that) specify the behavior described above.