Migrate render_gfm behavior to vanilla Javascript

The render_gfm behavior currently relies on jQuery. As we are trying to move away from jQuery, it would be beneficial to migrate this behavior to a vanilla Javascript version. We might be able to rewrite it in-place. Otherwise, we can consider creating a new behavior altogether, and migrate usages iteratively, TBD.

• Removing jQuery from app/assets/javascripts/behaviors/markdown/render_sandboxed_mermaid.js : #349779 (closed).
• Remove jQuery from Vue components that use GFM(Remove jQuery from GFM rendering (!106960 - merged))
• Remove jQuery completely from GFM (Completely remove jQuery from GFM (!107218 - merged))