Support custom import namespace for functions implemented in JavaScript

Can I specify a custom import namespace for JavaScript function?

```js
mergeInto(LibraryManager.library, {
napi_create_int32: function (env, value, result) {
// ...
},
napi_create_int32__sig: 'ipip',
napi_create_int32__namespace: 'napi' // <-- instead of 'env'
})
```

compile result

```js
WebAssembly.instantiate(/* ... */, {
napi: { napi_create_int32 }
})
```

Refs: https://github.com/nodejs/node/pull/49037