fix: node built-in modules handling

[NathanWalker]

• Node.js built-in modules (e.g., node:url) are now resolved by creating in-memory polyfill modules, registered and reused in a virtual registry, instead of writing polyfill files to disk.
• Added new tests to verify that built-in modules like node:url and optional modules are resolved correctly using the new in-memory approach.
• The pathToFileURL polyfill now encodes paths more accurately by replacing %2F with /, ensuring correct URL formatting.

[simensBat]

This change helped me.