Allow for optional keys in JSON objects

[blink1073]

Allows for interfaces of the form:

export
interface Foo extends JSONObject {
  bar?: string;
}

cf microsoft/TypeScript#13195

[vidartf]

Is this something that would be considered, or are we waiting to see if the TS issue gets resolved (there is an experimental PR since April this year)?

[sccolbert]

I'm worried about the unintended consequences this will have. It will certainly break some existing code, as evidenced by the PR.

[vidartf]

What about a PR that instead adds additional types PartialJSONObject etc? The existing objects could be cast directly to it.

[sccolbert]

@vidartf That would work.

[blink1073]

Closing in favor of #417