godoc: make line numbers unselectable

Insert line numbers via a ::before pseudo-element so you can't select
them when you copy text. See the code comment for more information.

Fixes golang/go#20077.

Change-Id: I24a5b17786a52c8107b4f830e824526ba03bc38d
Reviewed-on: https://go-review.googlesource.com/41418
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed