feat(build) don't build library on postinstall

Do it on the prepack step, so it gets built before we upload the build
artifact.

.github/workflows/ci.yml

         node-version: '14.x'
     - run: npm i -g npm@7
     - run: npm install
+    - run: npm run build
     - run: npm run lint
     - run: npm run test

.gitignore

 dist
 types/types-comparer/auto.json
 types/types-comparer/hand-crafted.json
+*.tgz

package-lock.json

     "": {
       "name": "lib-jitsi-meet",
       "version": "0.0.0",
-      "hasInstallScript": true,
       "license": "Apache-2.0",
       "dependencies": {
         "@jitsi/js-utils": "2.0.0",

package.json

     "webpack-cli": "4.9.0"
   },
   "scripts": {
+    "build": "webpack && tsc",
     "lint": "eslint .",
     "lint-fix": "eslint . --fix",
-    "postinstall": "webpack && tsc",
+    "prepack": "npm run build",
     "test": "karma start karma.conf.js",
     "test-watch": "karma start karma.conf.js --no-single-run",
     "validate": "npm ls",