[129/webpack] TMP: use npm preinstall step instead of prepare

We're experiencing what looks like
yarnpkg/yarn#7212 when we try to install
the bookreader repo through yarn. It says the prepare step on a
git:// referenced dependency (which is what we use) might be causing
this error (although this error doesn't happen at all when run on
@jbuckner's machine with that exact setup :/). Trying the `preinstall`
hook to see if that fixes things.

package.json

@@ -44,6 +44,7 @@
     ]
   },
   "scripts": {
+    "preinstall": "npm run build",
     "build": "npx webpack && npx sass ./src/css/BookReader.scss > ./BookReader/BookReader.css",
     "eslint": "eslint ./BookReader/BookReader.js",
     "lint": "eslint \"./**/*.js\"",