Update TypeScript target to match NodeJS 12

[andrewiggins]

Since the package.json specifies that the min NodeJS version is 12, I've updated the TypeScript to target es2017. The only change this has in the output is that async/await is no longer downleveled and is left as-is since NodeJS 12 supports async/await natively 🎉

I came across this while debugging locally in my node_modules and not having to jump through TypeScript's __awaiter helper would make this easier, at least for users like me 😅