Error code ambiguity

Over in Wasmtime, the CLI binary rewrites certain exit codes from WASI's proc_exit in order to avoid issues on Windows; see here. This makes it difficult to write WASI tests (e.g., in wasi-testsuite) that reliably emit the same exit code across operating systems. I discovered this in https://github.com/bytecodealliance/wasmtime/pull/5907 and will temporarily disable the tests on Windows.

I propose that we decide whether the exit code translation that Wasmtime currently does is something that all WASI engines should do. If so, then we can document that translation here and then the wasi-testsuite can either: a) create test runners that understand the exit code translation or b) write tests that avoid the ambiguity entirely.

cc: @sunfishcode, who has more context on this decision.

Start with the linked Wasmtime code in crates/wasi/src/lib.rs#L102-L107 and review the exit-code behavior described for Windows. Then examine the temporarily disabled WASI tests and the wasi-testsuite context. Done means the cross-platform translation policy is decided, documented, and its test-suite implications are clear.