Extract Alpine.js in script/test so the pre-commit hook works

The extraction moves from the Makefile's assets target into
script/assets. script/test runs it before the tests, so make test,
make check, script/check and the pre-commit hook all get the file on a
fresh clone; make assets, make build and make dev call the same script.
The README now says only that nothing downloads Alpine.js.

Model: opus-5-5

script/assets

@@ -0,0 +1,16 @@
+#!/bin/sh
+# script/assets: extract Alpine.js from its npm package tarball, committed
+# in 3p/, to static/js/alpine.min.js, where go:embed reads it. The
+# extracted file is not committed. script/test, make build and make dev run
+# this first.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    tar -xzOf 3p/alpinejs-3.14.9.tgz package/dist/cdn.min.js \
+        >static/js/alpine.min.js
+}
+
+main "$@"