Add platform field to Result, add TypeScript definition

[BendingBender]

Waiting for tsdjs/tsd#19

[sindresorhus]

Why are you adding a platform field? Users can just check process.platform themselves, or am I missing a use-case?

[BendingBender]

It's for the discriminated union type in TypeScript to work correctly.

This way, no null checks are needed, just a check for a platform. You can take a look at the TS tests, its visible there.

[sindresorhus]

Ah, I see. So there's no way of doing this with the NodeJS.Platform instead? Just feels weird exposing a field just for TS usage, but if there's no other way, I'm ok with it.

[sindresorhus]

@BendingBender This still has WIP: in the title, but it looks ready to me. Is there anything else left or is the title just outdated?

[BendingBender]

Waiting for tsdjs/tsd#19. Otherwise the expectError statements will fail in the test.

[sindresorhus]

Can you update this to use CommonJS-compatible syntax?

Also needs to be updated for f06f7e3

[BendingBender]

Waiting for tsdjs/tsd#20.