Add support for 3fg_drag libinput touchpad feature

libinput has implemented a three-finger drag feature for touchpads currently not supported by Sway.

Feature overview: https://wayland.freedesktop.org/libinput/doc/latest/drag-3fg.html

Implementation commit: https://gitlab.freedesktop.org/libinput/libinput/-/commit/1d9e307e2b92322a5102add55641f953d8e952fe

