Add a simple script to generate SRI hashes for our assets. (#24814)

package.json

@@ -55,6 +55,7 @@
     "docs-upload-preview": "build/upload-preview.sh",
     "docs-workbox-precache": "node build/workbox.js",
     "maintenance-dependencies": "ncu -a -x jquery && npm update && bundle update && shx echo \"Manually update assets/js/vendor/*, js/tests/vendor/* and .travis.yml\"",
+    "release-sri": "node build/generate-sri.js",
     "release-version": "node build/change-version.js",
     "release-zip": "cd dist/ && zip -r9 bootstrap-$npm_package_version-dist.zip * && shx mv bootstrap-$npm_package_version-dist.zip ..",
     "dist": "npm-run-all --parallel css js",
@@ -106,6 +107,7 @@
     "rollup-plugin-node-resolve": "^3.0.0",
     "shelljs": "^0.7.8",
     "shx": "^0.2.2",
+    "sri-toolbox": "^0.2.0",
     "stylelint": "^8.2.0",
     "stylelint-config-recommended-scss": "^2.0.0",
     "stylelint-config-standard": "^17.0.0",