Add result.all TypeScript definition

[ehmicky]

Fixes #342

While the all property is defined on the resolved value or rejected error (where it's a string or buffer), we forgot it on the return value (where it's a stream).

@tomsotte

[tomsotte]

As per merge-stream current implementation the mixed all stream that we create it's a PassThrough.
For execa purpose I believe it's correct to limit its type to a Readable stream.

[ehmicky]

Yes I think that it being a PassThrough is an implementation detail. From the user's perspective, it should be manipulated like stdout and stderr which are typed as Readable streams.

[ehmicky]

Fixed.