Improve the TypeScript types #86

I think the types could still be improved.

• Would be good to use unknown instead of any here: https://github.com/sindresorhus/conf/blob/bb24cfe32d39617a4b3983a92bd81596bd87c312/index.d.ts#L210 But it causes error on a valid usage:

    index.test-d.ts:123:13
      ✖  123:13  Argument of type "debug phase" is not assignable to parameter of type never.

• You cannot use dot-paths with a typed store: config.set('foo.bar', true);. We need to find a way to support that. Could possibly add support for config.set(['foo', 'bar'], true).
• Reduce duplication if you use both a typed store and the defaults option. Maybe using the const keyword.
• Could we make the types even stricter?
• Expose the type of the schema option, so it's easier to define it as a variable before passing it. Currently, you would have to do const schema = {[Key in keyof TypedStore]: Conf.Schema} = {…};. https://github.com/sindresorhus/conf/blob/bb24cfe32d39617a4b3983a92bd81596bd87c312/index.d.ts#L50

I'm open to other ideas on how to improve the types.