feat: add go.mod

[kaka19ace]

Hi, can i add go.mod file to the repo ？

For the Ref: https://golang.org/ref/mod#modules-overview

A module is identified by a module path, which is declared in a go.mod file...

[puellanivis]

This has come up before regularly (please look at the closed PRs dealing with go.mod).

The stance has been and remains:

• this package has no dependencies, and thus does not need a file to track dependencies.
• downstream users of this package do not need this file in order to use this package.

As such, there is no actual need for this file.

[joanlopez]

It's exactly what @puellanivis said. So, I close this issue. Thanks!