[VS Code] Add syntax highlighting in code blocks

User problem to solve

In VS Code, we have the ability for Duo Chat to interpret and generate code snippets. Currently, those snippets do not have syntax highlighting. Syntax highlighting would improve the readability of the code.

Proposal

1. When Duo Chat generates a code block in the response, it should have accurate syntax highlighting.
2. When the user clicks on Copy button, then the code should be copied to the clipboard without syntax highlighting. User's IDE is responsible for syntax highlighting per user configurations.