v1 release

I believe it would be beneficial to release `go-cmp` as `v1`.

Reasons:

1. The library is widely used, and I assume there’s no intention to introduce breaking changes while still in `v0`. If that’s the case, a `v1` release would formally acknowledge its stability.
2. A stable `v1` appears to be a prerequisite for resolving https://github.com/google/go-cmp/issues/338.
3. `go-cmp` is recommended by the [official Go test comments guide](https://go.dev/wiki/TestComments). Given this endorsement, it would be ideal for the package to be versioned in a way that ensures long-term stability (i.e., avoiding breaking changes without a `v2` release).
4. This will mitigate possible build failures if there if `go-cmp` is used by different Go test helper modules and they use different (incompatible) versions of `go-cmp`.