Fix diff-{language} highlighting #94

[sharavananpa]

References:

• Issues using diff- syntax #94
• Diff doesn't seem to work eleventy-base-blog#211

Basically, the diff-{language} identifier wasn't working.

Looks like the problem was in the PrismLoader.js where we set Prism.languages[ fullLanguageName ] to PrismDiff which will always be an empty object, since prism-diff.js contains just a IIFE that doesn't return anything and only add the diff loader object inside Prism.languages (global object).

So, I've set Prism.languages[ fullLanguageName ] to Prism.languages.diff which returns the proper grammar that is received in markdownSyntaxHighlightOptions.js to call the Prism.highlight(str, loader, language) method.

Note:

• I think we don't even have to set Prism.languages[ fullLanguageName ] since it will be created for us in the hooks that are run when calling the Prism.highlight() method. But I've refrained from directly returning the diff loader speculating it could have another utility and I didn't want to break things.
• There were existing test cases that covered this scenario but I think they got omitted since they had the .mjs file extension. Having these cases could've prevented the bug from getting in.
• Lastly I think the test files could use some refactoring. I'm interested in doing that. Do let me know if you want me to do that!

[zachleat]

Appreciate this PR so much! Yeah, if you want to take a swing at some test refactoring or general cleanup here, feel free!

[zachleat]

Shipping with v5.0.2