Use reference counting for unity loader hook

[alexqhj]

Implemented a reference-counting system for the useUnityLoader hook to track how many components are using the same Unity loader script.

• Prevents duplicate scripts by checking if the script already exists before creating a new one.
• Ensures proper cleanup, only removing the script when the last component referencing the script unmounts.

Why?

• Fixes a bug where multiple hook instances are pointed to the same script and causes errors when unmounting (eg. removing a script that another instance still needs).

[alexqhj]

Sorry I didn’t open a discussion beforehand. This fix was pretty urgent for our team so I went ahead and (hopefully) patched it. There might be a better approach, but I’m hoping this at least addresses the immediate issue. Would love your thoughts.