Go syntax - Separate ControlFlow from Keywords

The syntax definition for Go did not differentiate ControlFlow-specific Keywords from other Keywords, unlike those of other languages.

This change should make the code easier to read and more consistent with other commonly used languages (I previously made the same change for Rust).

After thinking about this case, I decided that the "go" keyword was better as a normal keyword, rather than ControlFlow, but I can change that if people disagree.

I also could not find any autotest for Go. I did not try to add one, since this is not the point of this MR, but I can do it if requested.