feat: Add markdown to code suggestions supported langs

Description

Add Markdown to the list of supported languages for code suggestions. 

Related Issues

Part of gitlab#443365 (closed)

Related to gitlab-org/editor-extensions&13

How has this been tested?

1. Follow steps to run gdk with the ai gateway
2. Run the VS Code extension locally (step 1)
3. Create a file with the .md extension
4. Note that code suggestions work/don't

before	after
before_no_markdown	tbd

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap