add function index for script.json (WASM)

[i-saint]

It seems you already know this, but in WASM the address of a function is actually the index of its reference table. However, this can be confusing if you're not familiar with the WASM specifications. This PR aims to make it easy by adding a function index to the script.json.
For instance, a function with index 123 will be $func123 at runtime.

[IamSanjid]

Hi, this is actually what I was looking for months, I didn't know those method addresses were reference indecies of the elem segment, is there any documnet I can find about why/how unity decides that these functions would be referenced inside of the elem segment? TBH this actually makes sense if you think how the javascript calls sense function sheesh never thought about it thanks.