Config option for language version

[peterebden]

We currently don't have any way to indicate this - we basically run go tool compile without specifying a language version. This means there's no way of opting into features like the new for loop variable scoping. This is presumably going to continue to be a thing in future.

I think this is probably just a config field for the plugin, equivalent to the field in the go.mod file.

[peterebden]

This may not be correct. On testing it looks like go tool compile is defaulting to its current version if not told otherwise - so we may not need to do anything to opt in (although arguably this would still be useful to hold back new features)