Improve `render` functions return types

[art049]

Hey, thanks a lot for this library!

Is your feature request related to a problem? Please describe. When using the lib with typescript, calling render will return the union string | Promise<string> | void whatever the call arguments are. This is painful as using the library results always results in an unnecessary cast:

const rendered = Eta.render(template, {}) as string

From what I see in the code, the same problem applies to renderAsync, renderFile and renderFileAsync

Describe the solution you'd like It should be possible to overload the function types to allow better-returned types

[shadowtime2000]

Will look more into this 👍