Better formatting of result messages

[chungyc]

GitHub code scanning only supports text messages in result objects, and the rules which can include Markdown-formatted messages are supposed to be unchanging and orthogonal to individual results, so I had resigned myself to the very poor formatting that GitHub does for the messages explaining how an individual code snippet could be fixed.

However, https://github.com/orgs/community/discussions/35440 suggests that GitHub does support some Markdown formatting for text messages in result objects, so it might be possible to format the messages better for GitHub. The most difficult part in all this might be finding out exactly what limited set of Markdown formatting is supported by GitHub for code scanning messages.