devel/go-mode.el: new port

This is go-mode, the Emacs mode for editing Go code.

It is a complete rewrite of the go-mode that shipped with Go 1.0.3 and
before, and was part of Go 1.1 until Go 1.3. Beginning with Go 1.4,
editor integration will not be part of the Go distribution anymore,
making this repository the canonical place for go-mode.

PR: 276329