feature(types): add type definitions

[demurgos]

Why

See my blog post on the benefits of type definitions.
signal-exit is extremely popular since it's used as a dependency of most code coverage tools. Writing down the public API in a strict format is a good way to specify the contracts of the API.

This PR is part of the effort to improve code coverage tools for Node as part of node-tooling group. I am going over the dependencies and checking them.

What

Add type definitions for signal-exit and link them in package.json. From a semver point of view this is a feature. Even if it's mostly there for documentation purposes, it enables TS consumers to types for this lib.

[demurgos]

CI failure is due to Appveyor and Node 5, this is not caused by this PR.

[demurgos]

Bump: @isaacs @bcoe @profnandaa

[profnandaa]

I'd prefer that we deal with the major issues first (for moving this package forward), before we come to types.

[demurgos]

@profnandaa Which major issues?

[profnandaa]

@demurgos - sorry, must have mixed up with spawn-child, this is the very last PR on this project as per the plan; got it now.

[demurgos]

Yes, the signal-exit code is good. I just want to add these types to enable static checks and code completion in the consumers.

[demurgos]

Ping @bcoe @isaacs

[bcoe]

@demurgos is this still something you'd like to land, would love to work with you to get some of the work you've done in the past over the finish line.

[demurgos]

I had less time for open-source at the start of the year but with the release of Node 12, I am catching up and am definitely OK with finishing what was started 🙂

I'll be on the node-tooling slack.

[bcoe]

bump, this would still be pretty cool to land -- or could we land it on DefinitelyTyped?

[TooTallNate]

👍

[isaacs]

This will be superceded by v4 very shortly.

There are types on @types/signal-exit in the meantime.