Open files in the default editor

You can find what editor is assigned to a given file extension, and tell the editor to open files or folders.

Implementations for:

• macOS: uses CoreServices.framework, and has special-case workarounds for quirks of Xcode and Sublime Text 3.

• Unix: uses xdg-mime + gtk-launch and xdg-open as a fallback.

• Windows: not supported yet.