Use `go.work` in the same way as `go.sum`

[whi-tw]

Go workspaces were introduced in Go 1.18: https://go.dev/blog/get-familiar-with-workspaces

They list a go version, so we can use them in the same way as go.mod

[kennyp]

Thanks @whi-tw! This and the current behavior seem a bit at odds with #70 and the upstream ASDF sentiment. I'd love to come up with an approach that works for everybody, but I haven't sorted it out yet.

I'm open to ideas.

[kennyp]

Related to #99