Edit files on GitHub

[GregBrimble]

There seems to already be some half-setup GitHub integration in the Workspace:

• https://github.com/microsoft/vscode/blob/f8a735348564a9b87988a270bc7d4a2118bddd3b/resources/web/code-web.js#L338-L346
• https://github.com/microsoft/vscode/blob/f8a735348564a9b87988a270bc7d4a2118bddd3b/resources/web/code-web.js#L380-L395

We should get this working so that users can open a GitHub repository with VSCode Workers, and then, ideally, edit files in memory, until they commit and push. Lacking that, we could use CF Workers Git Gateway to directly edit files.