Support for line numbers in syntax highlighting blocks

[brettaufheber]

The attributes "linenums" and "highlight" as described here are currently ignored by the highlight.js extension.

Also, another attribute is required to start code snippets at a specific line number.

A treeProcessor can be implemented to solve the issue.

const language = block.getAttribute('language', 'nohighlight');
const useLineNumbers = block.getAttribute('linenums', null) === 'linenums';
const startFrom = block.getAttribute('start-from', 1);
const highlights = block.getAttribute('highlight', null);