Support /io/in[/path/to/folder] imports to filesystem

We should allow users to drag-and-drop files or click a "Import Files..." button and select them from the filesystem. If they have specified a path, then we should create it (`mkdirp`) and put the files there. Otherwise we should use `/` by default.

There are a bunch of pre-existing drag-and-drop and file import type libs/components we can probably use. There's also code in Brackets that can do some of this.