set comments to two slashes only for go.mod

[ArtAndreev]

According to Go Modules Reference: Lexical elements:

Comments start with // and run to the end of a line. /* */ comments are not allowed.

Also go toolchain throws an error if comment of wrong format appears:

$ go mod tidy
go: errors parsing go.mod:
<path>/go.mod:<line>:<column>: mod files must use // comments (not /* */ comments)

This PR sets comments to // only in go.mod (gomod filetype).

Same can be set for go.work file too (Lexical elements), but currently vim-go doesn't provide filetype plugin for it.

[bhcleek]

Thank you