Can't Load Gist

GitHub switched the default branch for gists to `main` a while ago, but `fiddle-core`'s built in gist loading will try to check out `master`.

Simple solution would be to use `main` when cloning the gist, but old gists will still be using `master`. Most robust solution is probably to port the code in Fiddle's [`src/renderer/remote-loader.ts`](https://github.com/electron/fiddle/blob/b1fd2001913fe2529a4fdbdf191bc19313216cd1/src/renderer/remote-loader.ts) and use Octokit to grab the gist instead of just git.