Add `wasip2` and rename `wasi` to `wasip1`

WASI 0.2 (`wasip2`) launched [2024-01-25](https://bytecodealliance.org/articles/WASI-0.2) with ABI changes that make it incompatible with WASI 0.1 (`wasip1`).

I propose we;
* Add `//cpu:wasip2`
* Rename `//cpu:wasi` to `//cpu:wasip1`, leaving behind an alias for backward compatibility.

This mirrors what Rust ([tier 3](https://blog.rust-lang.org/2024/04/09/updates-to-rusts-wasi-targets/), [tier 2](https://blog.rust-lang.org/2024/11/26/wasip2-tier-2/)) and LLVM have done.

This is an existing issue requesting constraints for ABI, #38. It may make sense to keep `//cpu:wasi` as it is and instead have a separate ABI constraint.

Another consideration is CPU feature sets, since `wasip2` is more than just an ABI change. In some ways it is similar to micro-architecture levels (e.g. `x86-64-v4`, `ARMv9.0-A`), which bares similarity to what #13 is requesting.

Start by reviewing the existing `//cpu:wasi` constraint and the related ABI and CPU-feature discussions in issues #38 and #13, then compare the referenced Rust and LLVM target changes. Done means the project has an agreed approach for `wasip2`, the `wasi` compatibility alias or replacement, and any separate ABI or feature constraints.

Written by the indexing model from the issue text.