Turn-on TS esModuleInterop

- newish feature I didn't know about: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html
- also added `esnext.asnyciterable` to lib (may just add all of `esnext`)

tsconfig.json

@@ -6,14 +6,15 @@
         "moduleResolution": "node",
         "jsx": "react",
         "noImplicitAny": false,
+        "esModuleInterop": true,
         "sourceMap": true,
         "emitDecoratorMetadata": true,
         "experimentalDecorators": true,
         "skipDefaultLibCheck": true,
         "skipLibCheck": true,
         "strictNullChecks": false,
         "outDir": "tmp",
-        "lib": ["es2017", "dom"],
+        "lib": ["es2017", "dom", "esnext.asynciterable"],
         "paths": {
             "abstract-leveldown": ["node_modules/abstract-leveldown"]
         }