This adds code to recognize "wasm32-wasip1", "wasm32-wasip2", and
"wasm32-wasip3" as explicit targets, and adds a deprecation warning when
the "wasm32-wasi" target is used, pointing users to the "wasm32-wasip1"
target.
Fixes#165344.
I'm filing this as a draft PR for now, as I've only just now proposed to
make this change in #165344.