proposal: cmd/compile: add GOEXPERIMENT=loopvar

[rsc]

@dr2chase has been working on an implementation of the for loop scoping change in discussion #56010. Before we get to a formal proposal, it would be good to make the code available for people to try in their own code. I propose we add the change behind a GOEXPERIMENT=loopvar setting, so that developers can judge the effect in their own code, including in the Go 1.21 release (by setting the environment variable and recompiling).

[gopherbot]

Change https://go.dev/cl/411904 mentions this issue: cmd/compile: experimental loop iterator capture semantics change