JSON.stringify can return undefined

[WearyMonkey]

According to Note 2 of the JSON.stringify spec:

The undefined value is not rendered.

https://www.ecma-international.org/ecma-262/6.0/#sec-json.stringify

After this change, the return type of JSON.stringify will be string | undefined if the value parameter extends undefined.

Fixes #18879

[msftclas]

All CLA requirements met.

[cevek]

I may be undefined only if ReplacerFunction is specified

[RyanCavanaugh]

I think it'd be preferable to write this as overloads

stringify(x: object | number | boolean | string | null): string;
stringify(x: unknown): string | undefined;

This way invocations of stringify(any) maintain their current behavior of returning string (this is going to be far too large of a breaking change otherwise), but identifiably-unsafe calls like stringify(object | undefined) correctly include undefined in the return type.

[RyanCavanaugh]

Closing due to continued CI failure