[dev.go2go] src/const: set extensionId to golang.go2go

Change-Id: I915a9b3c9668e9102b6b4a55ef1097452a7978a7
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/241618


Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ahmed W. <oneofone@gmail.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>