feat: add --panic-unwind flag to build and test commands

[guybedford]

This adds a new --panic-unwind flag to wasm-pack build and wasm-pack test so projects can opt in to building with -Cpanic=unwind instead of the default panic=abort.

Rust's default panic strategy on wasm32-unknown-unknown is abort, which terminates the WebAssembly instance on any panic. With nightly + -Zbuild-std, projects can now rebuild std with unwinding enabled, allowing bindings layers (e.g. wasm-bindgen's catch-unwind support) to convert panics into catchable JavaScript exceptions. Doing this manually requires plumbing several flags and ensuring the nightly toolchain has the right components — --panic-unwind collapses that into a single flag.

Worth noting up front: this is a build-tool-level enabler. It produces a .wasm compiled with unwinding semantics. The actual conversion of caught panics into JS exceptions is the responsibility of the consuming bindings layer; wasm-pack does not generate runtime shims for this.

When --panic-unwind is passed, cargo is invoked with:

cargo +nightly build --lib -Z build-std=std,panic_unwind …
RUSTFLAGS="${existing} -Cpanic=unwind"

Any pre-existing RUSTFLAGS value is preserved.

• Adds --panic-unwind to BuildOptions and TestOptions
• Threads it through to cargo_build_wasm / cargo_build_wasm_tests
• Adds check_nightly_prerequisites() in src/build/wasm_target.rs that auto-installs the nightly toolchain, rust-src component, and nightly wasm32-unknown-unknown target via rustup when missing
• Skips the stable step_check_rustc_version and step_check_for_wasm_target when --panic-unwind is set (the stable toolchain isn't invoked in that mode)
• Documents the flag in docs/src/commands/build.md and docs/src/commands/test.md

Before:

cmd.arg("build").arg("--lib");
// stable toolchain only

After:

if panic_unwind {
    cmd.arg("+nightly");
}
cmd.arg("build").arg("--lib");
// later:
if panic_unwind {
    cmd.arg("-Z").arg("build-std=std,panic_unwind");
    cmd.env("RUSTFLAGS", format!("{existing} -Cpanic=unwind"));
}

cargo fmt --check is clean and cargo clippy introduces no new warnings. The build::* integration tests all pass locally (cargo test --test all -- --test-threads=1 build:: — 16/16). The default code path (without the flag) is unchanged.

Related: #737 (panic_immediate_abort, distinct from this — that's a size flag).

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