proposal: cmd/go: allow setting runtime godebugs in go.mod / go:debug directives

### Proposal Details

Now `asyncpreemptoff=1` cannot be specified by `//go:debug`:

```
examples\test\main.go:1:1: invalid //go:debug: unknown //go:debug setting "asyncpreemptoff"
```

However, there are many situations where disabling async preemption is required as a workaround to make applications work correctly:

* https://github.com/golang/go/issues/36981
* https://github.com/golang/go/issues/48059
* https://github.com/golang/go/issues/57442
* https://github.com/golang/go/issues/64781
* https://github.com/golang/go/issues/68485
* https://github.com/golang/go/issues/71242
* https://github.com/hashicorp/terraform/issues/27350

I propose to enable to specify `asyncpreemptoff=1` as `//go:debug` and `godebug` section in go.mod.

Note that it is still possible to specify it by `-ldflags="-X=runtime.godebugDefault=asyncpreemptoff=1"`, but this is pretty hacky.