…ION (#1578)

Two bugs broke `curl https://wasm-bindgen.github.io/wasm-pack/installer/init.sh -sSf | sh`:

1. The arg-parsing block introduced in 15e4f4f used bash arrays
   (`ORIG_ARGS=("$@")` / `"${ORIG_ARGS[@]}"`), so any non-bash POSIX
   shell (dash, ash, BusyBox sh) failed with a syntax error. This is
   issue #1527.

2. `docs/_installer/build-installer.rs` does a literal `$VERSION` ->
   `vX.Y.Z` text replacement on the script. Since 15e4f4f also added
   real `$VERSION` shell expansions (`if [ -z "$VERSION" ]`, the
   `case "$VERSION"`, etc.), every one of those got mangled in the
   published artifact, e.g. `if [ -z "v0.14.0" ]` and
   `VERSION="vv0.14.0"`. The runtime --version / VERSION-env override
   has been silently broken in every published release since.

This commit:

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

Verified syntax-clean and behaviourally correct under dash, /bin/sh,
and bash for piped invocation (`curl ... | sh`), `sh -s -- --version
X`, env override, and arg passthrough.

Supersedes #1550, closes #1527.