Add strong typing of .opts() using generics

[shadowspawn]

Pull Request

Problem

Unable to specify .opts() return type in TypeScript client code.

Issue: #1536

Original Pull Request: #1537 (manually moved code between branches, with co-author credit on commit)

Solution

Add generics so can call like:

interface MyCheeseOption {
  cheese: string;
}
const myCheeseOption = program.opts<MyCheeseOption>();

ChangeLog

• add client typing of .opts() return type using TypeScript generics

[abetomo]

👍

[shadowspawn]

For some experimental strong inferred typing see: https://github.com/commander-js/extra-typings