add TypeScript definition

package.json

@@ -3,6 +3,7 @@
   "version": "0.9.3",
   "description": "FFmpeg WebAssembly version",
   "main": "src/index.js",
+  "types": "src/index.d.ts",
   "directories": {
     "example": "examples"
   },
@@ -50,6 +51,7 @@
     "@babel/core": "^7.12.3",
     "@babel/preset-env": "^7.12.1",
     "@ffmpeg/core": "^0.8.2",
+    "@types/emscripten": "^1.39.4",
     "babel-loader": "^8.1.0",
     "chai": "^4.2.0",
     "cors": "^2.8.5",