Add file icons for Markdoc files

[markolech]

Add file icons for Markdoc files

Closes markdoc/markdoc#558

This PR adds visual file icons for Markdoc files in VSCode's file explorer to improve developer experience and file identification.

Changes

• Add official Markdoc logo icons for light and dark themes
• Configure file icons for .mdoc, .markdoc, and .markdoc.md extensions
• Update .vscodeignore to include icons in extension package

Implementation

• Uses the official Markdoc logo from https://markdoc.dev/logo.svg
• Follows VSCode's language icon API standards
• Icons appear in file explorer, tabs, and file dialogs

Testing

• ✅ Extension builds successfully
• ✅ Icons display correctly in both light and dark themes
• ✅ All Markdoc file extensions supported

[cla-assistant[bot]]

All committers have signed the CLA.