Support default language for code highlighting

[jrnewton]

highlight js for some reason will highlight certain words when in "auto" mode (eg when no explicit language is provided). See this comment for an example, https://github.com/jrnewton/github-readme-to-html/issues/9#issuecomment-1224150417

@geelen would prefer if 'text' language was used so that no words are highlight.

This ticket will a) raise the change in hljs, b) implement the usage via the hljs extension for this project.