Optional properties

[thesunny]

The purpose of the PR is to make object() types with optional() values appear as optional properties on an object. e.g.

// given
const struct = object({ a: optional(string()) })

// it should return this type (the `?` being the key change)
type X = {
  a?: string | undefined
}

There is also a related change to package.json which adds a script watch:types. The purpose of this script is so that when there are changes to code, the TypeScript type definitions get updated. During the normal watch, type changes are not reflected. A better solution may be to add build:types as part of the watch script instead of in a separate watch:types script.

Stylistically, the type helpers are near the object code. There aren't any existing type helpers (at least in the code I saw) so the choice to locate the type helpers near the code is arbitrary. It may be preferable to place them in utils; however, the code isn't used anywhere else (ie. it's not shared) so I opted to put it right above the object code. Let me know if you prefer it elsewhere.

[thesunny]

@ianstormtaylor

This passes all tests on my computer but ci is failing. It's unclear as to why this is happening.

[ianstormtaylor]

This is amazing, thank you @thesunny! I'll try restarting the CI to see if that fixes it.