fix(installer): make init.sh POSIX-compatible and stop mangling $VERSION

[guybedford]

This fixes two bugs that together break curl https://wasm-bindgen.github.io/wasm-pack/installer/init.sh -sSf | sh. Resolves #1527, supersedes #1550.

The arg-parsing block introduced in 15e4f4f used bash arrays (ORIG_ARGS=("$@") / "${ORIG_ARGS[@]}"), so any POSIX-only shell (dash, ash, BusyBox sh) failed with a syntax error — the symptom reported in #1527.

While investigating, I noticed a second, latent bug: docs/_installer/build-installer.rs does a literal $VERSION → vX.Y.Z text replacement on the published script. Since 15e4f4f also added real $VERSION shell expansions, every one of those got mangled in the published artifact. The current published init.sh contains, for example:

if [ -z "v0.14.0" ]; then
    VERSION="latest"
fi
# ...
case "v0.14.0" in
    v*) ;;
    *) VERSION="vv0.14.0" ;;
esac

so the runtime --version / VERSION= override has been silently broken in every release since.

This PR:

• switches the build-time placeholder to @@WASM_PACK_VERSION@@ so it cannot collide with real shell variable references (init.sh and index.html updated).
• rewrites init.sh as POSIX sh: no arrays, no local, scans args via for _arg in "$@" without consuming them so the original arg list still flows through to wasm-pack-init.
• preserves the intended precedence: explicit VERSION= env > --version X arg > build-time baked version > "latest" (resolved via the GitHub releases API).

Before (published artifact, paraphrased):

ORIG_ARGS=("$@")              # bash-only
# ...
set -- "${ORIG_ARGS[@]}"      # bash-only
if [ -z "v0.14.0" ]; then ... # broken by $VERSION substitution

After:

DEFAULT_VERSION="@@WASM_PACK_VERSION@@"   # replaced at build time
# ...
resolve_version() {
    if [ -n "${VERSION:-}" ]; then return; fi
    _prev=""
    for _arg in "$@"; do
        if [ "$_prev" = "--version" ]; then VERSION="$_arg"; return; fi
        _prev="$_arg"
    done
    VERSION="${DEFAULT_VERSION:-latest}"
}

Verified syntax-clean (sh -n, dash -n, bash -n) and behaviourally correct under dash, /bin/sh, and bash for:

• piped invocation (curl ... | sh)
• curl ... | sh -s -- --version X
• VERSION=X curl ... | sh
• extra non-version args passed through to the installer