Use `guessEmbeddedLanguages` in fenced code block when there is no language specified

Use [guessEmbeddedLanguages](https://github.com/search?q=repo%3Ashikijs%2Fshiki%20guessEmbeddedLanguages&type=code) in fenced code block when there is no language specified. This will result in a much better user experience when the language is not explicitly specified, and should probably be set as default fallback.