Improve Javascript Examples

[fboucquez]

At the moment, javascript examples are compiled versions of the typescript. This is great because we only need to maintain the typescript examples.

The problem is that the "compiled" javascript is too low level. Is it possible to tune the typescript compiler so the code keeps some of newer javascript syntax?

This will get worse if we start using syntax like async-await for the issue #324

https://docs.symbolplatform.com/guides/account/getting-the-amount-of-assets-sent-to-an-account.html

I think just updating the target version of typescript to ES2017 would do the trick

https://github.com/nemtech/symbol-docs/blob/main/source/resources/examples/typescript/tsconfig.json#L4

Also, the typescript compilation is not in package.json, it should be added as a command.