update param of `BinaryenRefNull` to take `BinaryenHeapType` (#8981)

According to the [Wasm
spec](https://webassembly.github.io/spec/core/syntax/instructions.html#reference-instructions),
the `(ref.null <ht>)` instruction takes a heap type as an argument, not
a reference type. This PR updates `BinaryenRefNull` to align with that.

The parameter is changed from a `BinaryenType` to a `BinaryenHeapType`
and removes the nullable assertion. This puts the onus on the caller to
make sure they are passing in a correct type.

This is a breaking change and would require callers to be more careful
about their arguments, but it provides for a more spec-aligned and
predictable API.

### Problem Statement
As an example of a problem it solves: say you’re using GC and you define
your own custom heap type (example in JS):
```wasm
(type $NumberUnion (struct
	(field $tag   i8)
	(field $int   i64)
	(field $float f64)
))
```
```js
const tb = new binaryen.TypeBuilder();
tb.setStructType(0, [
	{type: binaryen.i32, packedType: "i8"},
	{type: binaryen.i64, packedType: "notPacked"},
	{type: binaryen.f64, packedType: "notPacked"},
]);
const heaptypes = tb.buildAndDispose();
const ht_NumberUnion = heaptypes[0];
```
and then you want to create a null-ref expression `(ref.null
$NumberUnion)` in your module code:
```js
const mod = new binaryen.Module();
mod.ref.null(ht_NumberUnion); // error: `BinaryenRefNull` currently only accepts ref types
```
To fix, you have to create your own nullable ref type first:
```js
mod.ref.null(binaryen.getTypeFromHeapType(ht_NumberUnion, true));
```

With this PR’s proposed change, you can now just call `BinaryenRefNull`
passing in your heap type.