Added type annotations for a few glue code functions

[RunDevelopment]

I added type annotations to a few commonly-used JS glue code functions. This makes it a lot easier to understand the glue, because I have TS to help me out.

Those type annotations also make it easier to verify the correctness of glue. And just to prove that: TS found 2 minor bugs in the JS glue code of our tests:

Fixes moved to #4192.

1. ~debugString (this is the JS glue code version of format!("{:?}", value)) had a bug where it checked for a regex mismatch incorrectly. This would lead to dereferencing null at runtime, which throws a TypeError.~

2. ~_assertClass returned instance.ptr. Here's the full code of the function:~

   function _assertClass(instance, klass) {
       if (!(instance instanceof klass)) {
           throw new Error(`expected instance of ${klass.name}`);
       }
       return instance.ptr;
   }
   

   ~Returning instance.ptr is weird for 2 reasons:~

   • ~It's not used anywhere in the code. _assertClass is only used in one place and its return value is ignored.~
   • ~There is no ptr field. The author probably meant __wbg_ptr to get the internal pointer of a JS object.~

   ~So I just removed the return statement and instance.ptr.~

---

Aside from that, I did one additional change: I inline lTextDecoder:

// before

const lTextDecoder = typeof TextDecoder === 'undefined' ? (0, module.require)('util').TextDecoder : TextDecoder;

let cachedTextDecoder = new lTextDecoder('utf-8', { ignoreBOM: true, fatal: true });

// after

/** @type {TextDecoder} */
let cachedTextDecoder = new (typeof TextDecoder === 'undefined' ? (0, module.require)('util').TextDecoder : TextDecoder)('utf-8', { ignoreBOM: true, fatal: true });

This just made typing easier, but the expression something to get used to...