Feature Request - install a global.json file too to ensure all projects, inc future projects use the same framework version

[CarlSargunar]

As you're building a solution file and adding it, it would be quite nice to have a global.json in the root

This could also be tied to specific SDK versions for specific umbraco versions.

I wouldn't mind having a go at this if I can find the time to do it, but I'll prob have to do it after November, as I'm doing a few talks before then