Markdown syntax highlight in @description param decorators in VSCode

# Is your feature request related to a problem? Please describe.
The @description param decorator accepts markdown, and if I hover over a param with a description the markdown is rendered correctly. But when I write down the string, the editor dows not provide markdown syntax highlight

# Describe the solution you'd like
Markdown syntax highlight inside the value of the @description decorator
```bicep
// 👇 here
@description('''*some* description''')
param someParam string
```

It'd be a nice addition to also add this to bicep fenced code blocks in various markdown parsers (starting with the one in GitHub)

Start by examining how VSCode handles syntax highlighting for Bicep @description decorator values, then review the requested Bicep fenced code block behavior in GitHub and other Markdown parsers. Done means Markdown syntax is highlighted inside the decorator value without breaking Bicep highlighting, with the fenced-block addition addressed or explicitly scoped.

Written by the indexing model from the issue text.