feat(editor): enable GFM syntax for Markdown mode

[bajrangCoder]

Closes: #2921

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with the Markdown loader and its focused GFM coverage aligned.

Summary

This PR changes the built-in Markdown language loader from the default CommonMark parser to CodeMirror’s GFM-enabled Markdown language.

• Enables syntax support for GFM tables, task lists, and strikethrough.
• Adds parser-level coverage confirming those constructs are recognized through the registered Markdown mode.

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