fix(build,ts) clean before building

This makes sure we don't leave stray files behind if source JS files are removed.

package.json

     "webpack-cli": "4.9.0"
   },
   "scripts": {
-    "build": "export LIB_JITSI_MEET_COMMIT_HASH=$(git rev-parse --short HEAD 2>/dev/null) && webpack && tsc",
+    "build": "export LIB_JITSI_MEET_COMMIT_HASH=$(git rev-parse --short HEAD 2>/dev/null) && webpack && tsc --build --clean && tsc",
     "lint": "eslint .",
     "lint-fix": "eslint . --fix",
     "prepack": "npm run build",