Rename `wasm32-wasi-preview1-threads` to `wasm32-wasip1-threads`
This commit renames the current `wasm32-wasi-preview1-threads` target to `wasm32-wasip1-threads`. The need for this rename is a bit unfortunate as the previous name was chosen in an attempt to be future-compatible with other WASI targets. Originally this target was proposed to be `wasm32-wasi-threads`, and that's what was originally implemented in wasi-sdk as well. After discussion though and with the plans for the upcoming component-model target (now named `wasm32-wasip2`) the "preview1" naming was chosen for the threads-based target. The WASI subgroup later decided that it was time to drop the "preview" terminology and recommends "pX" instead, hence previous PRs to add `wasm32-wasip2` and rename `wasm32-wasi` to `wasm32-wasip1`.
So, with all that history, the "proper name" for this target is different than its current name, so one way or another a rename is required. This PR proposes renaming this target cold-turkey, unlike `wasm32-wasi` which is having a long transition period to change its name. The threads-based target is predicted to see only a fraction of the traffic of `wasm32-wasi` due to the unstable nature of the WASI threads proposal itself.
While I was here I updated the in-tree documentation in the target spec file itself as most of the documentation was copied from the original WASI target and wasn't as applicable to this target.
Also, as an aside, I can at least try to apologize for all the naming confusion here, but this is hopefully the last WASI-related rename.
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