Upgrade @ffmpeg/core to v0.8.4

Author: jeromewu

package-lock.json

@@ -50,7 +50,7 @@
   "devDependencies": {
     "@babel/core": "^7.12.3",
     "@babel/preset-env": "^7.12.1",
-    "@ffmpeg/core": "^0.8.3",
+    "@ffmpeg/core": "^0.8.4",
     "@types/emscripten": "^1.39.4",
     "babel-loader": "^8.1.0",
     "chai": "^4.2.0",