Use Chroma fallback when no language is specified

[helfper]

Currently, when using code fences without language specified (and guessSyntax is disabled), no CSS class or style is added to pre/code. I think it would be more useful to use the fallback lexer instead -- like it happens when guessSyntax is used and the language isn't identified -- as it would use a consistent fore/background color without special CSS handling.

[helfper]

This actually may be in https://github.com/yuin/goldmark-highlighting

[jmooring]

Related to, or duplicate of, #8824.