Add command for go mod init {moduleName}

[suzmue]

When starting a new Go project in VSCode. The first step is to create a go.mod file if there isn't already one. This currently requires navigating to the terminal and running go mod init moduleName. It would be easier to have a command that you can run from the command prompt that will activate the Go extension, and create the go.mod in the workspace folder.

This command could be easily suggested / run by the extension to help users when they are encountering errors because they are missing go.mod.

[gopherbot]

Change https://golang.org/cl/312092 mentions this issue: package.json: add command to run 'go mod init'