Add a new function isStringify()

[samchon]

export function isStringify<T>(input: T): string | null;
export function createIsStringify<T>(): (input: T) => string | null;

Composite function combined with TSON.is() and TSON.stringify().

If succeeded to pass the type checking, JSON string would be returned.

Otherwise, null value would be returned.

[samchon]

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