Commit the Alpine.js tarball in 3p/ and extract it at build time (closes #345)

The build no longer downloads Alpine.js. Its npm package tarball is committed as 3p/alpinejs-3.14.9.tgz, byte for byte the file script/fetch-assets downloaded, with the sha256 that script pinned. script/assets (make assets) extracts package/dist/cdn.min.js to the ignored static/js/alpine.min.js; script/test runs it, so make test, make check, the pre-commit hook and the Dockerfile get the file with no network access, and make build, run and dev run it too.

Removed: script/fetch-assets, its Dockerfile step, static/vendor.sha256 and static/vendor_test.go. The README describes the new flow.

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 "$@"