fix: Prevent crash when global URL constructor is not defined

[wearzdk]

The Problem

Currently, superjson's internal isURL check (payload instanceof URL) will throw a TypeError: Right-hand side of 'instanceof' is not an object in JavaScript environments where the global URL constructor is not available.

This issue occurs in environments like:

• WeChat Mini Programs
• Other non-standard JavaScript runtimes

This prevents users in these growing ecosystems from using superjson without applying workarounds like global polyfills or custom configuration overrides.

The Solution

This PR adds a guard clause (typeof URL !== 'undefined') before the instanceof check. By leveraging short-circuiting, this ensures that instanceof URL is only evaluated when the URL constructor actually exists.

• In standard environments (Browsers, Node.js): The behavior is completely unchanged.
• In environments without URL: The check now correctly and safely returns false instead of crashing.

This is a minimal, non-breaking change that significantly improves the library's robustness and compatibility.

[wearzdk]

Hi, I've just added a test following your suggestion to simulate an environment without URL. It no longer throws errors now. Looking forward to your review!

[Skn0tt]

uh-oh, could you take a look at the linter?

[wearzdk]

Okay, I've fixed the lint issues.