Use Highlight.php instead of Prism.js for server-side rendering of Code block's syntax highlighting

[westonruter]

• Builds on top of commits from Adhere to ESLint/PHPCS and allow for translations #12. See changes unique to this PR: https://github.com/mkaz/code-syntax-block/pull/13/files/ff5e498..7a8d213
• Replace Prism.js with highlight.php to do server-side rendering instead of client-side rendering; this improves frontend performance and allows highlighted code to be used in non-JS contexts, such as when JS is disabled or a page is being served with the AMP plugin. Syntax-highlighted code is stored in a transient.
• Make block markup backwards-compatible with code block.
• Add “Auto-detect” option as default language in block settings. Any existing core Code blocks will automatically get syntax highlighting added to them.
• Ensure blocks created prior to this PR are successfully re-parsed.

Screenshots:

[mehigh]

Tested it today, works really well. Thanks @westonruter for the contribution!

[mkaz]

I really appreciate the effort you put in for this, but I'm going to keep this plugin as front-end only using Prism. For a server-side rendering plugin, please feel free to fork from this PR, if you do let me know and I can add a link in the readme for the alternate implementation.

I just want to keep this fairly low maintenance and simple, the less things in it the less likely it will break down the line; the reason I did #23 and removing build and other steps, fewer dependencies and moving parts.