Support for deploying from a folder in a repo with the deploy to deta button

[Wallvon]

See title.

This would be useful if I, for example, were to make a repo with a frontend and backend folder, and I want people to be able to deploy the backend on a Deta micro with the deploy to Deta button. Currently, this is not possible, however, it would be accomplishable by for example looking for /tree/branch_here/folder_name in the repo URL. (So it becomes https://go.deta.dev/deploy?repo=your-repo-url/tree/branch_name/folder_name.)