style: don't assume HTML structure of rendered markdown in searchResultMatch

Created by: attfarhan

Avoid relying on the structure of the response of our markdown renderer when rendering code blocks. Instead, we just want the text of any result wrapped in a code block, so we can pass it to the syntax highlighter.

Should mean that we avoid issues where syntax highlighting does not occur because we fail to detect a code block, such as https://github.com/sourcegraph/sourcegraph/issues/2325.