Fix TypeScript definitions to match actual module export

[a-skindzer]

Issue

The TypeScript definition file declares a named export:

export function phpUnserialize(phpstr: string): unserialized;

However, the actual JavaScript module exports a function as the default export.
This mismatch causes a runtime error when TypeScript users try to import and use the function:

import { phpUnserialize } from "phpunserialize"; // TypeScript thinks this works
phpUnserialize(data); // Runtime error: not a function

Solution:

Update the TypeScript definitions to represent the default export correctly:

export type unserialized = phpscalar | phparray;
declare function phpUnserialize(phpstr: string): unserialized;
export default phpUnserialize;

This allows the correct usage:

import phpUnserialize from "phpunserialize";
phpUnserialize(data); // Works correctly