Add partial json type that allows for optional keys

[vidartf]

Refactor of #303 to instead add new partial types that allow undefined value and/or missing keys ("Partial" types in TS jargon).

Some of the type names get a little long, but at least they are explicit.

[vidartf]

Note: While the logic in JSONExt changed to support the new type, I do not think the behavior for the old type should differ (potentially a minor performance hit). If this is not the case, or the performance hit is not acceptable, we could duplicate that into PartialJSONExt as well.

[blink1073]

LGTM!

[sccolbert]

This can break third party code which is using the functions as type assertion checks, since it's changing the return type. Instead, let's add overloads for each of these functions.

[vidartf]

I think the current code only adds overloads, and leaves the return type untouched. It does change the argument type though.

[vidartf]

Do you have an example that currently works, but will fail after this change? It would make it a lot easier to get this right.

[vidartf]

Note: With the current code, these narrowings are in effect:

// Same as before:
let a = [] as JSONValue;
let b = [] as ReadonlyJSONValue;
if (JSONExt.isArray(a)) {
  // a: JSONArray
}
if (JSONExt.isArray(b)) {
  // b: ReadonlyJSONArray
}

// New possibilities:
let c = [] as PartialJSONValue;
let d = [] as ReadonlyPartialJSONValue;
if (JSONExt.isArray(c)) {
  // c: PartialJSONArray
}
if (JSONExt.isArray(d)) {
  // d: ReadonlyPartialJSONArray
}