feature: implement expanderr

[fatih]

For more info see: https://github.com/stapelberg/expanderr

For vim, integration see: stapelberg/expanderr#3

I would like to see this is integrated into a new ife snippet which is then expanded automatically based on the return values using this tool (in addition to a command).

Not sure if it's possible yet but we need to experiment with it. We already have some snippets such as errn or errn, that expands to err checks. By using ife we could automatize the return values which I think would be a really helpful.

Also now with #1443 maybe we should think to not increase the number of commands and have a generic command for code generators? Example usage:

:GoCode expanderr
:GoCode fillstruct
...

There are many other ideas that will come in the future, so having a single command that supports all these commands would beneficial. Let's, however, discuss this in another issue.