Implement drag& drop attachments in Source Editor

Problem to solve

The markdown editor implemented in issues, epics, and comments as a textarea has functionality built in to help users write GLFM. One of these features is support for dragging and dropping a file into the textarea to attach it.

In order to even consider the Source Editor as a suitable replacement for the textarea, we need to support this workflow.

Proposal

Create an extension for Source Editor that can be loaded in the context of editing GLFM files and support dragging and dropping a file into the editor. We should show a custom UI overlay when dragging a supported file over the editor (the current implementation overlays a 📎 icon).