feat: infer result type correctly with TypeScript

[tomoto]

Greetings, it looks like some people (#126 for example) want the type definition to support the non-string results, and here is my contribution.

Basically I added the minimal fix to the type definition to infer the result type based on the output option, and the TypeScript test cases to validate the type checks successfully pass as intended. As long as the output option is statically known to the compiler (which hopefully would be the case in the typical use cases), the result type can be uniquely determined; otherwise, the applications are still responsible for asserting the expected type but they no longer need to insert the dirty as unknown at least.

I also removed the mention to DefinitelyTyped in the doc comment and the type definition generaiton script in package.json since they looked obsolete.

Hope this helps. Thank you very much.

[avoidwork]

Hi, thanks for preparing this; I missed the notification. Will get it merged shortly.

[avoidwork]

Unfortunately I need to force push the merge out; typescript/tsc is no longer usable from npx.

[tomoto]

Thank you for looking into this. OK, I will change package.json not to use npx. Actually I am not a huge fan of npx and did not know it could cause such an issue. Talk to you later. Thanks again!