Don't apply the 'literal' color to entire markdown code blocks (#2579)

### Before

![Screen Shot 2023-06-06 at 2 32 46
PM](https://github.com/zed-industries/zed/assets/326587/036f1da3-19c6-4979-adc6-c3133f8d63dc)

### After

![Screen Shot 2023-06-06 at 2 32 08
PM](https://github.com/zed-industries/zed/assets/326587/6d4729db-b76a-4052-847e-fd9de346f194)

Release Notes:

* Improved the syntax highlighting of fenced code blocks in markdown.