fix: Ship to NPM without node version restrictions (shaka-project#5253)

Before packaging for NPM, remove the "engines" section of package.json
to avoid unnecessary restrictions on node versions for projects that
simply depend on Shaka Player, but don't need to rebuild it. This is
accomplished with the clean-package tool:
https://github.com/roydukkey/clean-package

The "engines" section and its restrictions still make sense for Shaka
Player development, so it will not be removed from package.json in the
repo.

Closes shaka-project#5243

.npmignore

@@ -3,6 +3,7 @@
 .gitattributes
 .gitignore
 .release-please*
+*.backup
 *.pyc
 app-engine/
 build/

package.json

@@ -18,6 +18,7 @@
     "awesomplete": "^1.1.5",
     "babel-plugin-istanbul": "^6.1.1",
     "cajon": "^0.4.4",
+    "clean-package": "^2.2.0",
     "code-prettify": "^0.1.0",
     "codem-isoboxer": "^0.3.7",
     "color-themes-for-google-code-prettify": "^2.0.4",
@@ -96,12 +97,19 @@
   },
   "license": "Apache-2.0",
   "scripts": {
+    "prepack": "clean-package",
+    "postpack": "clean-package restore",
     "prepublishOnly": "python build/checkversion.py && python build/all.py --force"
   },
   "dependencies": {
     "eme-encryption-scheme-polyfill": "^2.1.1"
   },
   "engines": {
     "node": ">=14"
+  },
+  "clean-package": {
+    "remove": [
+      "engines"
+    ]
   }
 }