feat: add phase 6 js_library and ts_library bootstrap

tests/library_test/verify_bundle.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+bundle="$1"
+
+if [[ ! -s "${bundle}" ]]; then
+  echo "Expected bundled output to exist and be non-empty: ${bundle}" >&2
+  exit 1
+fi